feat: stop language fallback at English

Treating `en` the same as any other language is problematic as shown by
#12690. When the language precedence list contains entries after
English and we don't treat English specially, a lack of translations in
`en.po` (or a lack of `en.po`, once we delete it) results in
translations into those subsequent languages being displayed, instead of
the msgid, which is in English, and thus preferable.

By truncating the fallback list when we encounter `en`, this problem is
resolved. As it is implemented now, the `en.po` catalog is never used.
This is intended, as the plan is to delete it (#12745). In any case, its
translations are identical to the msgids modulo some fancy quotes.

While at it, also treat `LANGUAGE` values of `C` and `POSIX` as
referring to the English version of the messages.

Fixes #12690

Closes #12747