When I switch from gnome to cinnamon, then switch back to gnome, my mother-language keyboard option disappears (only English remains) and I have to add the language again from the settings! This bug is reproducable on my laptop
We need enough details to understand what you are doing.
How do you switch?
Step 1: Logout (using the GUI)
Step 2: Click the gear icon
Step 3: Choose Cinnamon
Step 4: Enter the password and login
Step 5: Finish whatever you wanna do with cinnamon
Step 6: Repeat steps (1-4) but choose gnome
Try raising a bug agaibst gnome. It’s should not lose setting when you login to another DE.
Unless they share config files???
Also what happens if you reboot before logging into gnome again?
I have posted the bug here as you said I should do so: Bug: Keyboard Layout Disappears when switching to another DE - Desktop - GNOME Discourse
Rebooting then choosing causes the same problem if the last session was in cinnamon.
Finally, It is unlikely that cinnamon and gnome share config files as cinnamon always remembers my layouts, unlike gnome. This means that although the layout gets, emoved in gnome, it doesn’t get removed in cinnamon.