Android Keyboard (AOSP) Spell Checker does only work with one language. If multiple (system) languages are selected and spell checker is set to use system languages, it will only use the first language. Word suggestions will change, when changing the keyboard language, but spell checker will not. Therefore everything written in the second language will be underlined in red and tapping them gives you suggestions in the first language.
Hi all, new member here!

Was the issue @reddek reported above ever replicated or resolved? Basically, I am observing exactly the same problem.
I just installed the latest version (17.1-20210117-NIGHTLY-a3xelte) on a Galaxy A3 (2016), and installation was really painless. I'm using English, French and German on a daily basis, and selected all 3 languages in the System > Languages and Input > Languages menu. I use the Android (AOSP) keyboard. I can switch the keyboard between languages, and the spellchecker while I'm typing checks in the correct (current) language. However, anything written in a message that is not German (the main language of the OS) will still be highlighted as a typo. So it seems there are two spellcheckers at work, one from the keyboard and another one for text already written, and the latter does not switch languages correctly. I have used several Android systems and this never happened to me before. Any idea how to fix this?
I'm happy to install different keyboards or spellcheckers, but so far have not succeeded in resolving this.
(I this thread is not for Samsung, but this is the only place I found so far that discussed this)