Most browsers return "ja" rather than "ja-JP" in navigator.language.
Because parse-locale expands "ja-JP" to ["ja_jp" "ja"] but keeps "ja"
as ["ja"], using "ja_jp" causes autodetection to fall back to English.
Normalize the Japanese locale identifier to "ja" and map jpn_JP.po to
translation.ja.js, matching other single-region locales like Ukrainian.
AI-assisted-by: gemini-3.8-flash
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