Fixes #21637 - fallback to default locale with I18n
This makes the I18n behavior closer to the gettext one, and prevents some ugly 'missing_translation' messages in the UI (for example in apipie, that uses the I18n backend for it's strings).
Related issues
Bug #21637: wrong <title> on http://sat630.example.com/apidoc/v2.<some langs>.html - contains html tag
Fixes #21637 - fallback to default locale with I18n
This makes the I18n behavior closer to the gettext one, and prevents
some ugly 'missing_translation' messages in the UI (for example in
apipie, that uses the I18n backend for it's strings).