KDE seems to often/usually use languages from the end of the regional settings list. I had in order British English, American English, Greek. I was usually getting popup/error/etc. messages in Greek. I removed it (just kept the alternative keyboard layout) but it's still happening.
STEPS TO REPRODUCE
1. Add languages in regional settings.
2. Do something to get a message.
3. See that it uses last, not first language on the list.
Uses last language on your list for messages. Konsole errors, also various non-KDE program tool-tips, and others did this.
Use first language on your list for messages.
Linux/KDE Plasma: Slackware64-current / 5.23.2
KDE Plasma Version: 5.23.2
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.3
*** This bug has been marked as a duplicate of bug 192019 ***