SUMMARY
After installation and login for the first time, the user encounters an empty Language window in Regional Settings, and adding several different languages there results a locale confusion affecting several applications in varying degrees (including Dolphin, Konsole, VLC Media Player, etc.)
In my case, I added Turkish after US English and UK English, and after logout and login, I found some apps trying to use Turkish as instead of English.

STEPS TO REPRODUCE
1. System Settings > Personalization: Regional Settings > Language
2. Add Turkish in the third row after American English (Default) and British English.
3. Check VLC, Konsole (e.g. executing: sudo apt purge), Dolphin (right-clicking a file or folder, selecting Properties, and seeing what's shown next to Type:)

OBSERVED RESULT
I saw Dolphin showing only file types in Turkish (everything else in English), Konsole partially displaying Turkish messages as output from terminal commands (like "sudo apt purge") probably due to some missing translations, and VLC entirely assuming current system language is Turkish - while the entire system remained in English as expected.

EXPECTED RESULT
Adding additional languages should not cause any such confusion and at any rate automatic replacement of the primary language.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 19.10
(available in About System)
KDE Plasma Version: 5.16.5
KDE Frameworks Version: 5.62.0
Qt Version: 5.12.4

ADDITIONAL INFORMATION
1. When I entered the first and second languages, there was no problem, but adding Turkish as the third language crashed the System Settings at first. So I could only do it in two steps.
2. I have returned to KDE after some years of separation, and I intend to stay this time thanks to the level of maturity reached as a result of great efforts. ;-)