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
LANG=en_CA.UTF-8
LANGUAGE=en_US:ko:fr
LC_CTYPE="en_CA.UTF-8"
LC_NUMERIC=en_CA.UTF-8
LC_TIME=en_CA.UTF-8
LC_COLLATE="en_CA.UTF-8"
LC_MONETARY=en_CA.UTF-8
LC_MESSAGES="en_CA.UTF-8"
LC_PAPER=en_CA.UTF-8
LC_NAME=en_CA.UTF-8
LC_ADDRESS=en_CA.UTF-8
LC_TELEPHONE=en_CA.UTF-8
LC_MEASUREMENT=en_CA.UTF-8
LC_IDENTIFICATION=en_CA.UTF-8
LC_ALL=

[copperplate:~]$ locale -a
C
C.UTF-8
en_AG
en_AG.utf8
en_AU.utf8
en_BW.utf8
en_CA.utf8
en_DK.utf8
en_GB.utf8
en_HK.utf8
en_IE.utf8
en_IL
en_IL.utf8
en_IN
en_IN.utf8
en_NG
en_NG.utf8
en_NZ.utf8
en_PH.utf8
en_SG.utf8
en_US.utf8
en_ZA.utf8
en_ZM
en_ZM.utf8
en_ZW.utf8
fr_BE.utf8
fr_CA.utf8
fr_CH.utf8
fr_FR.utf8
fr_LU.utf8
POSIX