I think the app I used was called “more physical keyboard layouts” or something like that. It’s for connecting physical keyboards though. Never felt the need to have it for the virtual keyboard, since you have all of the weird letters behind a long press anyways.

Here’s a link to the EurKey website: https://eurkey.steffen.bruentjen.eu

For european languages I usually recommend learning using an ANSI keyboard and using the EurKey layout (I installed it on Linux/Mac/Win and Android for a hardware keyboard without any issues). That way you have a larger choice in custom keyboards and a lot of Keyboard shortcuts make more sense (because software usually seems to be written for ANSI keyboards).

But I’m not sure how that would go with Cyrillic.