It's the age of LLMs! Language has been solved! LLMs are great at both Czech and Polish. This problem is orders of magnitude easier. Why doesn't my keyboard even know these words exist?? Is there an Android keyboard that actually... knows basic forms of basic words?
I use Google Pinyin Input. Since it was discontinued in favor of (the much worse) GBoard, I have to keep a backup of the apk and sideload it onto new phones.
Google does not appear to think of input methods as something that should be convenient for the user to use. Not sure why.