The interface followed the system locale with no way to override it. Settings now
carry a language picker, stored as forced_lang in gui.json, with "System" listed
first so the choice stays undoable.
rust-i18n keeps the current locale in a process-wide atomic and t! reads it on
every lookup, so switching takes effect on the next frame without a restart.
Languages are listed by their own name rather than a translated one, since that
is what a reader looking for their language recognises. A test asserts every
shipped locale has such a name, so adding a translation without naming it fails
rather than silently showing a bare language code.
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
