Allow customizing the fallback language

[Denis Anuschewski]

This is a follow-up ticket for #28636, where I proposed changes that differ from the much better solution which came up in the analysis from Claude Paroz. So in order to clean up things the new approach with the underlying problem is now mentioned here:

Currently there is no way to explicitly specify a fallback language for both unsupported languages ( = not in settings.LANGUAGES) and missing literal translations in the translation files. This job is done by settings.LANGUAGE_CODE which controls a lot of other language-related features.

It would be useful to be able to control this behavior without touching settings.LANGUAGE_CODE which defines your project's installation-wide language preference. One scenario in which you would benefit from that is having a non-English project where you want to deliver English as fallback for languages that differ from your base language.

[Denis Anuschewski]

My pull request ​https://github.com/django/django/pull/9248 implements this feature by introducing a new setting LANGUAGE_FALLBACK. When untouched it doesn't change the default translation routine, but by setting your desired language here you can control the translation fallback.

[Carlton Gibson]

I'm going to close this as a duplicate of #28636. Better to keep all the discussion in the one place there.