I had been running happily with most locale variables set to en_CA.utf8 and with "American English" as the only Plasma translation. I installed "français" and made it the default, tested it, then installed Korean and made it the default, tested it, then made American English the default again.
Even after restarting, the command line is a hilarious Unicode salad.
[copperplate:~]$ apt full-upgrade
E: 잠금 파일 /var/lib/dpkg/lock-frontend 파일을 열 수 없습니다 - open (13: Permission non accordée)
E: Impossible d'obtenir le verrou de dpkg (/var/lib/dpkg/lock-frontend). Avez-vous les droits du superutilisateur ?
[copperplate:~]$ locale -a
Setting “LANGUAGE=en_US” (just one language) or “LANGUAGE=C:fr:ko” fixes the command line. I suppose that few packages have any “en” localizations, and rely on the fall-back “C” strings to serve English users.
Perhaps kcm_language should never put an English locale by itself in LANGUAGE; it should always follow it with C, as in “LANGUAGE=fr:en_CA:C:ko”.
*** This bug has been marked as a duplicate of bug 192019 ***