spell check for more than 1 language in Android
This is a known bug in Gboard: 
https://issuetracker.google.com/issues/72386511

I tested flesky and SwiftKey and they do at least a spell check for one language. 

Even if I only chose one system language and only one dedicated spell check language Gboard underlines nothing. In general this is the same for every App. Only Google Docs seems to have its own spell check.