Fix i18n fallback configuration conflicts with environment configurations

[akihikodaki]

This fixes a bug reported at #6841 (comment).

[ineffyble]

Is this comment still accurate? Seems like it should say something more like "make lookups for any locale fall back to 'en'"?

[akihikodaki]

No. Addressed in 7820893. Thanks for pointing out this.

[ineffyble]

To clarify how this works for my own benefit:

• Instance owner sets a default locale via environment variable
• This default locale is used for all the localised strings shown within Mastodon
• If a string is not available in this locale, Mastodon will instead use the English version of the string

In addition:

• If the default locale is not provided, or is unavailable or invalid, it is set to English

Does that all sound right?

[akihikodaki]

Yes. And it has been the expected behavior, so it is a bug if it doesn't work as you stated. (That's why this pull request is labelled bug here.)

[nightpool]

i just touched this code recently for cybrespace-specific locale changes, so i can confirm this is the correct change to make. thanks aki!

[nightpool]

(to be clear, the bug here is that we were setting .fallbacks = [:en] in application.rb, but it was getting overwritten later by the default .fallbacks = true in production.rb)