[imorland]

Fixes #4763 (Make the translation fallback locale configurable, currently hardcoded to en)

Changes proposed in this pull request:
Missing translations currently jump to English even when the forum has another default language. Use the existing Default Language setting and its parent locales before English, preserving the selected language's precedence. Refresh forum and admin translation bundles when the default changes, and preserve reference targets across regional fallbacks.

Reviewers should focus on:

• Regression tests failed before the corresponding fallback, cache-refresh, reference-context and empty-default fixes, and pass with them.

Screenshot
Before

After

Recording: changing the default language

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.