Set Up BookBrainz for Internationalization

BookBrainz is currently English-only, limiting its global reach. This project integrates i18next with BookBrainz's React SSR pipeline to enable full internationalization: locale-detection middleware (Accept-Language + cookie), a language selector in the navbar, English translation JSON files, ~620 strings migrated across ~120 files, and a BookBrainz project on translations.metabrainz.org so volunteer translators can contribute immediately — with no rebuild required per translation update.