TestLangCompleteness uses reflection to walk the i18n struct and fail
on any empty string field, catching incomplete translations at CI time.
Running it immediately found three missing strings in lang_jp.go
(playtime, timer.enabled, timer.disabled), which are now filled in.
CHANGELOG updated with i18n refactor, FR/ES languages, and the new test.
sys_language.go now contains only the i18n struct, Bead type, lookup
helpers, and a minimal getLangStrings dispatcher. All string data lives
in lang_en.go and lang_jp.go, making it straightforward to add a new
language by creating a single self-contained file.
