In KStars 3.7.7, the language cannot be changed any more, just like #497225 and #485944. The system language is used instead. Used "Hilfe - Sprache einrichten" (Help - Switch Application Language) to change the language but after a restart of KStars the settings are ignored and still the language of the OS is used. I tried different systems to reproduce the bug in KStars – Linux Mint 22.1, RPi 5 (self compiled version for Raspberry Pi OS Bookworm, 64-bit) and Windows 11, all with the same result.
We pushed a fix to stable-3.7.7 (and master). Please recompile and let us know if it is fixed.