Just chiming in, I’m also getting this issue in writer and recently posted a video recording of this happening in the github issue #273 link to my comment Spelling check language reset when typing · Issue #273 · ONLYOFFICE/DesktopEditors · GitHub
I’m running
Onlyoffice: version 8.3.1.25 (deb)
Operating System: Manjaro Linux
KDE Plasma Version: 6.3.2
KDE Frameworks Version: 6.11.0
Qt Version: 6.8.2
Kernel Version: 6.12.17-1-MANJARO (64-bit)
Graphics Platform: Wayland