bpo-31159: fix language switch regex on unknown yet built languages. … (#3051)

bpo-31159: fix language switch regex on unknown yet built languages. … (#3051)

* bpo-31159: fix language switch regex on unknown yet built languages. (#3039)

This fix a regex issue (a missing non-matching group around an 'or'
list) and the specific possible case where a translation is built but
not yet in known by the picker, but not explicitly listing possible
languages in the regex.
(cherry picked from commit 122081deef86174beee965be1207fa46ea23533d)

* bpo-31149: Doc: Add Japanese to the language switcher. (#3028)

(cherry picked from commit c82b7f332aff606af6c9c163da75f1e86514125e)