Register the 12 additional languages (fr, de, it, nl, pl, el, ja, ko, zh,
ru, tr, ar) as available UI locales so the language switcher and the API
accept them, keeping the set in lockstep with the ContentLanguage enum.
- config/languages.php lists all 15 UI locales with their native names.
- ContentLanguage::isRtl() drives the document `dir`; SetLocale shares it
to the Blade root and HandleInertiaRequests shares it to Inertia, and
app.ts mirrors it on SPA navigations so RTL locales lay out correctly.
- dayjs imports the 12 new locales so dates localize instead of falling
back to English.
- A LocalizationParityTest guards against key drift: every locale must
ship every base translation file with exactly the keys of lang/en.
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