Add an interface-language preference. It defaults to the system's
language on first run (sys-locale, native only) and is switched by hand
from the View tab, next to Dark Mode, or with app.language.english /
app.language.japanese over the control channel. It is saved in ui.json.
Follows the filmcraft house convention: a Language enum and a
table of English-to-Japanese strings with English fallback, so coverage
can grow incrementally. Japanese chrome renders without any bundled
font: the CJK fallback faces already appended to every family cover
kana and kanji.
The ribbon chrome all flows through the small_button/big_button/
toggle_button/split_button/icon_button helpers and menu_items, so
translating there localises the whole ribbon in one place; the language
of the frame is read from egui context data (i18n::current). Command
ids, file names and document text are never translated.
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