diff --git a/src/analyze.test.ts b/src/analyze.test.ts
index c73159d1fe..0d0a48d18e 100644
--- a/src/analyze.test.ts
+++ b/src/analyze.test.ts
@@ -10,7 +10,7 @@ import { runQueries } from "./analyze";
 import { setCodeQL } from "./codeql";
 import { Config } from "./config-utils";
 import * as count from "./count-loc";
-import { Language } from "./languages";
+import { KnownLanguage } from "./languages";
 import { getRunnerLogger } from "./logging";
 import { setupTests, setupActionsVars } from "./testing-utils";
 import * as util from "./util";
@@ -21,8 +21,8 @@ setupTests(test);
 // and correct case of builtin or custom. Also checks the correct search
 // paths are set in the database analyze invocation.
 test("status report fields and search path setting", async (t) => {
-  const mockLinesOfCode = Object.values(Language).reduce((obj, lang, i) => {
-    // use a different line count for each language
+  const mockLinesOfCode = Object.values(KnownLanguage).reduce((obj, lang, i) => {
+    // use a different line count for each KnownLanguage
     obj[lang] = i + 1;
     return obj;
   }, {});
@@ -35,13 +35,13 @@ test("status report fields and search path setting", async (t) => {
     const addSnippetsFlag = "";
     const threadsFlag = "";
     const packs = {
-      [Language.cpp]: [
+      [KnownLanguage.cpp]: [
         {
           packName: "a/b",
           version: clean("1.0.0")!,
         },
       ],
-      [Language.java]: [
+      [KnownLanguage.java]: [
         {
           packName: "c/d",
           version: clean("2.0.0")!,
@@ -49,7 +49,7 @@ test("status report fields and search path setting", async (t) => {
       ],
     };
 
-    for (const language of Object.values(Language)) {
+    for (const language of Object.values(KnownLanguage)) {
       setCodeQL({
         packDownload: async () => ({ packs: [] }),
         databaseRunQueries: async (
@@ -203,7 +203,7 @@ test("status report fields and search path setting", async (t) => {
 
   function verifyLineCounts(tmpDir: string) {
     // eslint-disable-next-line github/array-foreach
-    Object.keys(Language).forEach((lang, i) => {
+    Object.keys(KnownLanguage).forEach((lang, i) => {
       verifyLineCountForFile(path.join(tmpDir, `${lang}.sarif`), i + 1);
     });
   }
@@ -249,14 +249,14 @@ test("status report fields and search path setting", async (t) => {
         version: "2.0.0",
       },
     ];
-    for (const lang of Object.values(Language)) {
+    for (const lang of Object.values(KnownLanguage)) {
       t.deepEqual(readContents(`${lang}-queries-builtin.qls`), qlsContent);
       t.deepEqual(readContents(`${lang}-queries-custom-0.qls`), qlsContent);
       t.deepEqual(readContents(`${lang}-queries-custom-1.qls`), qlsContent2);
       const packSuiteName = `${lang}-queries-packs.qls`;
-      if (lang === Language.cpp) {
+      if (lang === KnownLanguage.cpp) {
         t.deepEqual(readContents(packSuiteName), qlsPackContentCpp);
-      } else if (lang === Language.java) {
+      } else if (lang === KnownLanguage.java) {
         t.deepEqual(readContents(packSuiteName), qlsPackContentJava);
       } else {
         t.false(
diff --git a/src/analyze.ts b/src/analyze.ts
index 422df5eea9..5df4b7b85f 100644
--- a/src/analyze.ts
+++ b/src/analyze.ts
@@ -8,7 +8,7 @@ import * as analysisPaths from "./analysis-paths";
 import { CODEQL_VERSION_COUNTS_LINES, getCodeQL } from "./codeql";
 import * as configUtils from "./config-utils";
 import { countLoc } from "./count-loc";
-import { isScannedLanguage, Language } from "./languages";
+import { isScannedLanguage, KnownLanguage, Language } from "./languages";
 import { Logger } from "./logging";
 import * as sharedEnv from "./shared-environment";
 import * as util from "./util";
@@ -124,7 +124,7 @@ async function createdDBForScannedLanguages(
     ) {
       logger.startGroup(`Extracting ${language}`);
 
-      if (language === Language.python) {
+      if (language === KnownLanguage.python) {
         await setupPythonExtractor(logger);
       }
 
diff --git a/src/config-utils.test.ts b/src/config-utils.test.ts
index 6a6ae6d300..1e82249f61 100644
--- a/src/config-utils.test.ts
+++ b/src/config-utils.test.ts
@@ -9,7 +9,7 @@ import * as sinon from "sinon";
 import * as api from "./api-client";
 import { getCachedCodeQL, setCodeQL } from "./codeql";
 import * as configUtils from "./config-utils";
-import { Language } from "./languages";
+import { KnownLanguage, Language } from "./languages";
 import { getRunnerLogger } from "./logging";
 import { setupTests } from "./testing-utils";
 import * as util from "./util";
@@ -299,7 +299,7 @@ test("load non-empty input", async (t) => {
 
     // And the config we expect it to parse to
     const expectedConfig: configUtils.Config = {
-      languages: [Language.javascript],
+      languages: [KnownLanguage.javascript],
       queries: {
         javascript: {
           builtin: [],
@@ -974,7 +974,7 @@ test("No detected languages", async (t) => {
   });
 });
 
-test("Unknown languages", async (t) => {
+test("Unknown, unsupported languages", async (t) => {
   return await util.withTmpDir(async (tmpDir) => {
     const languages = "rubbish,english";
 
@@ -998,7 +998,77 @@ test("Unknown languages", async (t) => {
     } catch (err) {
       t.deepEqual(
         err,
-        new Error(configUtils.getUnknownLanguagesError(["rubbish", "english"]))
+        new Error(
+          configUtils.getUnsupportedLanguagesError(["rubbish", "english"])
+        )
+      );
+    }
+  });
+});
+
+test("Unknown, supported languages", async (t) => {
+  return await util.withTmpDir(async (tmpDir) => {
+    const languages = "ql";
+
+    const codeQL = setCodeQL({
+      async resolveLanguages() {
+        return {
+          ql: ["/tmp/ql-for-ql-pack"],
+        };
+      },
+    });
+
+    let config: configUtils.Config = await configUtils.initConfig(
+      languages,
+      undefined,
+      undefined,
+      undefined,
+      undefined,
+      { owner: "github", repo: "example " },
+      tmpDir,
+      tmpDir,
+      codeQL,
+      tmpDir,
+      gitHubVersion,
+      sampleApiDetails,
+      getRunnerLogger(true)
+    );
+    t.deepEqual(config.languages, ["ql"]);
+  });
+});
+
+test("Partially supported languages", async (t) => {
+  return await util.withTmpDir(async (tmpDir) => {
+    const languages = "rubbish,ql";
+
+    const codeQL = setCodeQL({
+      async resolveLanguages() {
+        return {
+          ql: ["/tmp/ql-for-ql-pack"],
+        };
+      },
+    });
+    try {
+      await configUtils.initConfig(
+        languages,
+        undefined,
+        undefined,
+        undefined,
+        undefined,
+        { owner: "github", repo: "example " },
+        tmpDir,
+        tmpDir,
+        codeQL,
+        tmpDir,
+        gitHubVersion,
+        sampleApiDetails,
+        getRunnerLogger(true)
+      );
+      throw new Error("initConfig did not throw error");
+    } catch (err) {
+      t.deepEqual(
+        err,
+        new Error(configUtils.getUnsupportedLanguagesError(["rubbish"]))
       );
     }
   });
@@ -1044,7 +1114,7 @@ test("Config specifies packages", async (t) => {
       getRunnerLogger(true)
     );
     t.deepEqual(packs as unknown, {
-      [Language.javascript]: [
+      [KnownLanguage.javascript]: [
         {
           packName: "a/b",
           version: clean("1.2.3"),
@@ -1102,13 +1172,13 @@ test("Config specifies packages for multiple languages", async (t) => {
       getRunnerLogger(true)
     );
     t.deepEqual(packs as unknown, {
-      [Language.javascript]: [
+      [KnownLanguage.javascript]: [
         {
           packName: "a/b",
           version: clean("1.2.3"),
         },
       ],
-      [Language.python]: [
+      [KnownLanguage.python]: [
         {
           packName: "c/d",
           version: clean("1.2.3"),
@@ -1377,8 +1447,8 @@ parsePacksErrorMacro.title = (providedTitle: string) =>
 function invalidPackNameMacro(t: ExecutionContext<unknown>, name: string) {
   parsePacksErrorMacro(
     t,
-    { [Language.cpp]: [name] },
-    [Language.cpp],
+    { [KnownLanguage.cpp]: [name] },
+    [KnownLanguage.cpp],
     new RegExp(
       `The configuration file "/a/b" is invalid: property "packs" "${name}" is not a valid pack`
     )
@@ -1388,8 +1458,8 @@ invalidPackNameMacro.title = (_: string, arg: string) =>
   `Invalid pack string: ${arg}`;
 
 test("no packs", parsePacksMacro, {}, [], {});
-test("two packs", parsePacksMacro, ["a/b", "c/d@1.2.3"], [Language.cpp], {
-  [Language.cpp]: [
+test("two packs", parsePacksMacro, ["a/b", "c/d@1.2.3"], [KnownLanguage.cpp], {
+  [KnownLanguage.cpp]: [
     { packName: "a/b", version: undefined },
     { packName: "c/d", version: clean("1.2.3") },
   ],
@@ -1398,9 +1468,9 @@ test(
   "two packs with spaces",
   parsePacksMacro,
   [" a/b ", " c/d@1.2.3 "],
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "a/b", version: undefined },
       { packName: "c/d", version: clean("1.2.3") },
     ],
@@ -1410,16 +1480,16 @@ test(
   "two packs with language",
   parsePacksMacro,
   {
-    [Language.cpp]: ["a/b", "c/d@1.2.3"],
-    [Language.java]: ["d/e", "f/g@1.2.3"],
+    [KnownLanguage.cpp]: ["a/b", "c/d@1.2.3"],
+    [KnownLanguage.java]: ["d/e", "f/g@1.2.3"],
   },
-  [Language.cpp, Language.java, Language.csharp],
+  [KnownLanguage.cpp, KnownLanguage.java, KnownLanguage.csharp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "a/b", version: undefined },
       { packName: "c/d", version: clean("1.2.3") },
     ],
-    [Language.java]: [
+    [KnownLanguage.java]: [
       { packName: "d/e", version: undefined },
       { packName: "f/g", version: clean("1.2.3") },
     ],
@@ -1430,21 +1500,21 @@ test(
   "no language",
   parsePacksErrorMacro,
   ["a/b@1.2.3"],
-  [Language.java, Language.python],
+  [KnownLanguage.java, KnownLanguage.python],
   /The configuration file "\/a\/b" is invalid: property "packs" must split packages by language/
 );
 test(
   "invalid language",
   parsePacksErrorMacro,
-  { [Language.java]: ["c/d"] },
-  [Language.cpp],
+  { [KnownLanguage.java]: ["c/d"] },
+  [KnownLanguage.cpp],
   /The configuration file "\/a\/b" is invalid: property "packs" has "java", but it is not one of the languages to analyze/
 );
 test(
   "not an array",
   parsePacksErrorMacro,
-  { [Language.cpp]: "c/d" },
-  [Language.cpp],
+  { [KnownLanguage.cpp]: "c/d" },
+  [KnownLanguage.cpp],
   /The configuration file "\/a\/b" is invalid: property "packs" must be an array of non-empty strings/
 );
 
@@ -1496,8 +1566,8 @@ function parseInputAndConfigErrorMacro(
 parseInputAndConfigErrorMacro.title = (providedTitle: string) =>
   `Parse Packs input and config Error: ${providedTitle}`;
 
-test("input only", parseInputAndConfigMacro, {}, " c/d ", [Language.cpp], {
-  [Language.cpp]: [{ packName: "c/d", version: undefined }],
+test("input only", parseInputAndConfigMacro, {}, " c/d ", [KnownLanguage.cpp], {
+  [KnownLanguage.cpp]: [{ packName: "c/d", version: undefined }],
 });
 
 test(
@@ -1505,9 +1575,9 @@ test(
   parseInputAndConfigMacro,
   {},
   "a/b , c/d@1.2.3",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "a/b", version: undefined },
       { packName: "c/d", version: "1.2.3" },
     ],
@@ -1519,9 +1589,9 @@ test(
   parseInputAndConfigMacro,
   {},
   "  +  a/b , c/d@1.2.3 ",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "a/b", version: undefined },
       { packName: "c/d", version: "1.2.3" },
     ],
@@ -1533,9 +1603,9 @@ test(
   parseInputAndConfigMacro,
   ["a/b", "c/d"],
   "  ",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "a/b", version: undefined },
       { packName: "c/d", version: undefined },
     ],
@@ -1547,9 +1617,9 @@ test(
   parseInputAndConfigMacro,
   ["a/b", "c/d"],
   " e/f, g/h@1.2.3 ",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "e/f", version: undefined },
       { packName: "g/h", version: "1.2.3" },
     ],
@@ -1561,9 +1631,9 @@ test(
   parseInputAndConfigMacro,
   ["a/b", "c/d"],
   " +e/f, g/h@1.2.3 ",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   {
-    [Language.cpp]: [
+    [KnownLanguage.cpp]: [
       { packName: "e/f", version: undefined },
       { packName: "g/h", version: "1.2.3" },
       { packName: "a/b", version: undefined },
@@ -1586,7 +1656,7 @@ test(
   parseInputAndConfigErrorMacro,
   {},
   "c/d",
-  [Language.cpp, Language.csharp],
+  [KnownLanguage.cpp, KnownLanguage.csharp],
   /multi-language analysis/
 );
 
@@ -1595,7 +1665,7 @@ test(
   parseInputAndConfigErrorMacro,
   {},
   " + ",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   /remove the '\+'/
 );
 
@@ -1604,7 +1674,7 @@ test(
   parseInputAndConfigErrorMacro,
   {},
   " xxx",
-  [Language.cpp],
+  [KnownLanguage.cpp],
   /"xxx" is not a valid pack/
 );
 
diff --git a/src/config-utils.ts b/src/config-utils.ts
index c0881314f1..909946a5f7 100644
--- a/src/config-utils.ts
+++ b/src/config-utils.ts
@@ -673,8 +673,8 @@ export function getNoLanguagesError(): string {
   );
 }
 
-export function getUnknownLanguagesError(languages: string[]): string {
-  return `Did not recognise the following languages: ${languages.join(", ")}`;
+export function getUnsupportedLanguagesError(languages: string[]): string {
+  return `Does not support the following languages: ${languages.join(", ")}`;
 }
 
 /**
@@ -714,8 +714,10 @@ async function getLanguagesInRepo(
  * has been set, otherwise it is deduced as all languages in the repo that
  * can be analysed.
  *
- * If no languages could be detected from either the workflow or the repository
+ * If no supported languages could be detected from either the workflow or the repository
  * then throw an error.
+ * 
+ * The set of supported languages is defined by the `codeql resolve languages` command.
  */
 async function getLanguages(
   codeQL: CodeQL,
@@ -731,11 +733,11 @@ async function getLanguages(
     .filter((x) => x.length > 0);
   logger.info(`Languages from configuration: ${JSON.stringify(languages)}`);
 
+  const allSupportedLanguages = new Set(Object.keys(await codeQL.resolveLanguages()));
   if (languages.length === 0) {
     // Obtain languages as all languages in the repo that can be analysed
     languages = await getLanguagesInRepo(repository, apiDetails, logger);
-    const availableLanguages = await codeQL.resolveLanguages();
-    languages = languages.filter((value) => value in availableLanguages);
+    languages = languages.filter((value) => allSupportedLanguages.has(value));
     logger.info(
       `Automatically detected languages: ${JSON.stringify(languages)}`
     );
@@ -748,21 +750,21 @@ async function getLanguages(
   }
 
   // Make sure they are supported
-  const parsedLanguages: Language[] = [];
-  const unknownLanguages: string[] = [];
+  const supportedLanguages: Language[] = [];
+  const unsupportedLanguages: Language[] = [];
   for (const language of languages) {
     const parsedLanguage = parseLanguage(language);
-    if (parsedLanguage === undefined) {
-      unknownLanguages.push(language);
-    } else if (parsedLanguages.indexOf(parsedLanguage) === -1) {
-      parsedLanguages.push(parsedLanguage);
+    if (!allSupportedLanguages.has(parsedLanguage)) {
+      unsupportedLanguages.push(language);
+    } else if (supportedLanguages.indexOf(parsedLanguage) === -1) {
+      supportedLanguages.push(parsedLanguage);
     }
   }
-  if (unknownLanguages.length > 0) {
-    throw new Error(getUnknownLanguagesError(unknownLanguages));
+  if (unsupportedLanguages.length > 0) {
+    throw new Error(getUnsupportedLanguagesError(unsupportedLanguages));
   }
 
-  return parsedLanguages;
+  return supportedLanguages;
 }
 
 async function addQueriesFromWorkflow(
@@ -1071,7 +1073,7 @@ export function parsePacksFromConfig(
     if (!Array.isArray(packsArr)) {
       throw new Error(getPacksInvalid(configFile));
     }
-    if (!languages.includes(lang as Language)) {
+    if (!languages.includes(lang)) {
       throw new Error(getPacksRequireLanguage(lang, configFile));
     }
     packs[lang] = [];
@@ -1172,7 +1174,7 @@ function shouldCombinePacks(packsInput?: string): boolean {
 function combinePacks(packs1: Packs, packs2: Packs): Packs {
   const packs = {};
   for (const lang of Object.keys(packs1)) {
-    packs[lang] = packs1[lang].concat(packs2[lang] || []);
+    packs[lang] = packs1[lang]?.concat(packs2[lang] || []);
   }
   for (const lang of Object.keys(packs2)) {
     if (!packs[lang]) {
diff --git a/src/count-loc.test.ts b/src/count-loc.test.ts
index 3a32ab6e1a..eb2cb69cef 100644
--- a/src/count-loc.test.ts
+++ b/src/count-loc.test.ts
@@ -3,7 +3,7 @@ import * as path from "path";
 import test from "ava";
 
 import { countLoc } from "./count-loc";
-import { Language } from "./languages";
+import { KnownLanguage } from "./languages";
 import { getRunnerLogger } from "./logging";
 import { setupTests } from "./testing-utils";
 
@@ -14,7 +14,7 @@ test("ensure lines of code works for cpp and js", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     [],
     [],
-    [Language.cpp, Language.javascript],
+    [KnownLanguage.cpp, KnownLanguage.javascript],
     getRunnerLogger(true)
   );
 
@@ -29,7 +29,7 @@ test("ensure lines of code works for csharp", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     [],
     [],
-    [Language.csharp],
+    [KnownLanguage.csharp],
     getRunnerLogger(true)
   );
 
@@ -43,7 +43,7 @@ test("ensure lines of code can handle undefined language", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     [],
     [],
-    [Language.javascript, Language.python, "hucairz" as Language],
+    [KnownLanguage.javascript, KnownLanguage.python, "hucairz" as KnownLanguage],
     getRunnerLogger(true)
   );
 
@@ -72,7 +72,7 @@ test("ensure lines of code can handle includes", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     ["../../src/testdata"],
     [],
-    [Language.javascript],
+    [KnownLanguage.javascript],
     getRunnerLogger(true)
   );
 
@@ -88,7 +88,7 @@ test("ensure lines of code can handle empty includes", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     ["idontexist"],
     [],
-    [Language.javascript],
+    [KnownLanguage.javascript],
     getRunnerLogger(true)
   );
 
@@ -102,7 +102,7 @@ test("ensure lines of code can handle exclude", async (t) => {
     path.join(__dirname, "../tests/multi-language-repo"),
     [],
     ["**/*.py"],
-    [Language.javascript, Language.python],
+    [KnownLanguage.javascript, KnownLanguage.python],
     getRunnerLogger(true)
   );
 
diff --git a/src/count-loc.ts b/src/count-loc.ts
index 549f07d703..6d0c6ee9be 100644
--- a/src/count-loc.ts
+++ b/src/count-loc.ts
@@ -1,19 +1,19 @@
 import { LocDir } from "github-linguist";
 
-import { Language } from "./languages";
+import { KnownLanguage, Language } from "./languages";
 import { Logger } from "./logging";
 
 // Map from linguist language names to language prefixes used in the action and codeql
-const linguistToMetrics: Record<string, Language> = {
-  c: Language.cpp,
-  "c++": Language.cpp,
-  "c#": Language.csharp,
-  go: Language.go,
-  java: Language.java,
-  javascript: Language.javascript,
-  python: Language.python,
-  ruby: Language.ruby,
-  typescript: Language.javascript,
+const linguistToMetrics: Record<string, KnownLanguage> = {
+  c: KnownLanguage.cpp,
+  "c++": KnownLanguage.cpp,
+  "c#": KnownLanguage.csharp,
+  go: KnownLanguage.go,
+  java: KnownLanguage.java,
+  javascript: KnownLanguage.javascript,
+  python: KnownLanguage.python,
+  ruby: KnownLanguage.ruby,
+  typescript: KnownLanguage.javascript,
 };
 
 const nameToLinguist = Object.entries(linguistToMetrics).reduce(
@@ -24,7 +24,7 @@ const nameToLinguist = Object.entries(linguistToMetrics).reduce(
     obj[name].push(key);
     return obj;
   },
-  {} as Record<Language, string[]>
+  {} as Record<KnownLanguage, string[]>
 );
 
 /**
diff --git a/src/database-upload.test.ts b/src/database-upload.test.ts
index de55a0245f..8c61eba2f4 100644
--- a/src/database-upload.test.ts
+++ b/src/database-upload.test.ts
@@ -10,7 +10,7 @@ import * as apiClient from "./api-client";
 import { setCodeQL } from "./codeql";
 import { Config } from "./config-utils";
 import { uploadDatabases } from "./database-upload";
-import { Language } from "./languages";
+import { KnownLanguage } from "./languages";
 import { Logger } from "./logging";
 import { RepositoryNwo } from "./repository";
 import { setupActionsVars, setupTests } from "./testing-utils";
@@ -36,7 +36,7 @@ const testApiDetails: GitHubApiDetails = {
 
 function getTestConfig(tmpDir: string): Config {
   return {
-    languages: [Language.javascript],
+    languages: [KnownLanguage.javascript],
     queries: {},
     pathsIgnore: [],
     paths: [],
diff --git a/src/init-action.ts b/src/init-action.ts
index 44402a688a..5faaa35705 100644
--- a/src/init-action.ts
+++ b/src/init-action.ts
@@ -21,7 +21,7 @@ import {
   installPythonDeps,
   runInit,
 } from "./init";
-import { Language } from "./languages";
+import { KnownLanguage } from "./languages";
 import { getActionsLogger } from "./logging";
 import { parseRepositoryNwo } from "./repository";
 import {
@@ -172,7 +172,7 @@ async function run() {
     );
 
     if (
-      config.languages.includes(Language.python) &&
+      config.languages.includes(KnownLanguage.python) &&
       getRequiredInput("setup-python-dependencies") === "true"
     ) {
       try {
diff --git a/src/languages.test.ts b/src/languages.test.ts
index 5837c91f8d..cdb9bac66f 100644
--- a/src/languages.test.ts
+++ b/src/languages.test.ts
@@ -1,7 +1,7 @@
 import test from "ava";
 
 import {
-  Language,
+  KnownLanguage,
   isScannedLanguage,
   isTracedLanguage,
   parseLanguage,
@@ -12,18 +12,18 @@ setupTests(test);
 
 test("parseLanguage", async (t) => {
   // Exact matches
-  t.deepEqual(parseLanguage("csharp"), Language.csharp);
-  t.deepEqual(parseLanguage("cpp"), Language.cpp);
-  t.deepEqual(parseLanguage("go"), Language.go);
-  t.deepEqual(parseLanguage("java"), Language.java);
-  t.deepEqual(parseLanguage("javascript"), Language.javascript);
-  t.deepEqual(parseLanguage("python"), Language.python);
+  t.deepEqual(parseLanguage("csharp"), KnownLanguage.csharp);
+  t.deepEqual(parseLanguage("cpp"), KnownLanguage.cpp);
+  t.deepEqual(parseLanguage("go"), KnownLanguage.go);
+  t.deepEqual(parseLanguage("java"), KnownLanguage.java);
+  t.deepEqual(parseLanguage("javascript"), KnownLanguage.javascript);
+  t.deepEqual(parseLanguage("python"), KnownLanguage.python);
 
   // Aliases
-  t.deepEqual(parseLanguage("c"), Language.cpp);
-  t.deepEqual(parseLanguage("c++"), Language.cpp);
-  t.deepEqual(parseLanguage("c#"), Language.csharp);
-  t.deepEqual(parseLanguage("typescript"), Language.javascript);
+  t.deepEqual(parseLanguage("c"), KnownLanguage.cpp);
+  t.deepEqual(parseLanguage("c++"), KnownLanguage.cpp);
+  t.deepEqual(parseLanguage("c#"), KnownLanguage.csharp);
+  t.deepEqual(parseLanguage("typescript"), KnownLanguage.javascript);
 
   // Not matches
   t.deepEqual(parseLanguage("foo"), undefined);
@@ -32,21 +32,21 @@ test("parseLanguage", async (t) => {
 });
 
 test("isTracedLanguage", async (t) => {
-  t.true(isTracedLanguage(Language.cpp));
-  t.true(isTracedLanguage(Language.java));
-  t.true(isTracedLanguage(Language.csharp));
+  t.true(isTracedLanguage(KnownLanguage.cpp));
+  t.true(isTracedLanguage(KnownLanguage.java));
+  t.true(isTracedLanguage(KnownLanguage.csharp));
 
-  t.false(isTracedLanguage(Language.go));
-  t.false(isTracedLanguage(Language.javascript));
-  t.false(isTracedLanguage(Language.python));
+  t.false(isTracedLanguage(KnownLanguage.go));
+  t.false(isTracedLanguage(KnownLanguage.javascript));
+  t.false(isTracedLanguage(KnownLanguage.python));
 });
 
 test("isScannedLanguage", async (t) => {
-  t.false(isScannedLanguage(Language.cpp));
-  t.false(isScannedLanguage(Language.java));
-  t.false(isScannedLanguage(Language.csharp));
+  t.false(isScannedLanguage(KnownLanguage.cpp));
+  t.false(isScannedLanguage(KnownLanguage.java));
+  t.false(isScannedLanguage(KnownLanguage.csharp));
 
-  t.true(isScannedLanguage(Language.go));
-  t.true(isScannedLanguage(Language.javascript));
-  t.true(isScannedLanguage(Language.python));
+  t.true(isScannedLanguage(KnownLanguage.go));
+  t.true(isScannedLanguage(KnownLanguage.javascript));
+  t.true(isScannedLanguage(KnownLanguage.python));
 });
diff --git a/src/languages.ts b/src/languages.ts
index a696908c7f..b4190e907b 100644
--- a/src/languages.ts
+++ b/src/languages.ts
@@ -1,5 +1,12 @@
-// All the languages supported by CodeQL
-export enum Language {
+/**
+ * A lowercase string representing a language.
+ */
+export type Language = Lowercase<string>;
+
+/**
+ *  All the languages known to be supported by CodeQL
+ */ 
+export enum KnownLanguage {
   csharp = "csharp",
   cpp = "cpp",
   go = "go",
@@ -11,20 +18,20 @@ export enum Language {
 
 // Additional names for languages
 const LANGUAGE_ALIASES: { [lang: string]: Language } = {
-  c: Language.cpp,
-  "c++": Language.cpp,
-  "c#": Language.csharp,
-  typescript: Language.javascript,
+  c: KnownLanguage.cpp,
+  "c++": KnownLanguage.cpp,
+  "c#": KnownLanguage.csharp,
+  typescript: KnownLanguage.javascript,
 };
 
 // Translate from user input or GitHub's API names for languages to CodeQL's names for languages
-export function parseLanguage(language: string): Language | undefined {
+export function parseLanguage(language: string): Language {
   // Normalise to lower case
   language = language.toLowerCase();
 
   // See if it's an exact match
-  if (language in Language) {
-    return language as Language;
+  if (language in KnownLanguage) {
+    return language;
   }
 
   // Check language aliases
@@ -32,14 +39,14 @@ export function parseLanguage(language: string): Language | undefined {
     return LANGUAGE_ALIASES[language];
   }
 
-  return undefined;
+  return language;
 }
 
 export function isTracedLanguage(language: Language): boolean {
   return (
-    ["cpp", "java", "csharp"].includes(language) ||
+    [KnownLanguage.cpp, KnownLanguage.java, KnownLanguage.csharp].includes(language as any) ||
     (process.env["CODEQL_EXTRACTOR_GO_BUILD_TRACING"] === "on" &&
-      language === Language.go)
+      language === KnownLanguage.go)
   );
 }
 
diff --git a/src/tracer-config.test.ts b/src/tracer-config.test.ts
index eae544844a..c62814a256 100644
--- a/src/tracer-config.test.ts
+++ b/src/tracer-config.test.ts
@@ -5,7 +5,7 @@ import test from "ava";
 
 import { setCodeQL } from "./codeql";
 import * as configUtils from "./config-utils";
-import { Language } from "./languages";
+import { KnownLanguage } from "./languages";
 import { setupTests } from "./testing-utils";
 import {
   concatTracerConfigs,
@@ -18,7 +18,7 @@ setupTests(test);
 
 function getTestConfig(tmpDir: string): configUtils.Config {
   return {
-    languages: [Language.java],
+    languages: [KnownLanguage.java],
     queries: {},
     pathsIgnore: [],
     paths: [],
@@ -49,7 +49,7 @@ test("getTracerConfigForLanguage - minimal setup", async (t) => {
     const result = await getTracerConfigForLanguage(
       codeQL,
       config,
-      Language.javascript
+      KnownLanguage.javascript
     );
     t.deepEqual(result, { spec: "abc", env: { foo: "bar" } });
   });
@@ -89,7 +89,7 @@ test("getTracerConfigForLanguage - existing / critical vars", async (t) => {
     const result = await getTracerConfigForLanguage(
       codeQL,
       config,
-      Language.javascript
+      KnownLanguage.javascript
     );
     t.deepEqual(result, {
       spec: "abc",
@@ -309,7 +309,7 @@ test("getCombinedTracerConfig - return undefined when no languages are traced la
   await util.withTmpDir(async (tmpDir) => {
     const config = getTestConfig(tmpDir);
     // No traced languages
-    config.languages = [Language.javascript, Language.python];
+    config.languages = [KnownLanguage.javascript, KnownLanguage.python];
 
     const codeQL = setCodeQL({
       async getTracerEnv() {