fix(l10n): make translation factory caches app- and locale-aware

[joshtrichards]

• Resolves: # (possibly some weird language/locale mismatch bugs)

Summary

Don't let the number of changed lines scare you; the bulk are either mechanical variable name changes or comment updates/removals.

Improve OC\L10N\Factory caching and clean up related internals.

Changes

• cache resolved request language per app context
• cache L10N instances per app, language, and locale
• deduplicate discovered languages before caching
• lazily build the language membership map used by languageExists()
• harden findLanguageFromLocale() for null / empty input
• clean up internal naming, comments, code duplication, and docblocks

Why

Factory::get() previously cached L10N instances only by app and language, which could reuse the same instance across different locales.

This also makes language lookup caching more explicit and avoids duplicate language entries when app and theme translations overlap.

Testing

• see TODO

TODO

• Updated / added tests that cover:
  • locale-specific L10N instance caching
  • lazy language membership-map construction
  • rebuilding the membership map from cached available languages
  • null / empty input handling in findLanguageFromLocale()

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI