Review the logic for the notification messages and the messages wording

[afercia]

Splitting this out from #46

See the TL;DR part starting from #46 (comment)

Basically, when changing language, WordPress downloads the core language packs but doesn't download the plugin/theme ones. These are only downloaded when users go to the Updates page and click "Update Translations".

In this scenario, right after users changed language and before they update (which may happen days later) the notification messages will be misleading.

Details

The logic for the messages uses is_textdomain_loaded() which return false because the new language .mo file doesn't exist yet, thus the logic fails to provide users with the correct message.

For example, switching to Deutsch (Sie) renders the message:

You're using WordPress in German (Formal). While Yoast SEO has been translated to German (Formal) for 96%, it's not been shipped with the plugin yet. You can help! Register at Translating WordPress to help complete the translation to German (Formal)!

Instead, it shouldn't render any notification.

One more example: switching to Dutch formal renders the message:

You're using WordPress in Dutch (Formal). While Yoast SEO has been translated to Dutch (Formal) for 54%, it's not been shipped with the plugin yet. You can help! Register at Translating WordPress to help complete the translation to Dutch (Formal)!

Instead, it should render (in Dutch):

As you can see, there is a translation of this plugin in Dutch (Formal). This translation is currently 54% complete. We need your help to make it complete and to fix any errors. Please register at Translating WordPress to help complete the translation to Dutch (Formal)!

Options

This should be improved upstream in WordPress. However, we could try to improve the messages. Or, inform users they need to go to the Updates page and update the translations.

[afercia]

See also #5

[afercia]

Created https://core.trac.wordpress.org/ticket/47108 for the WordPress core part.