I have English, French and Spanish keyboards on my S10.
It used to be that I'd swipe the space bar to select a language and everything would then happen in that language, including predictive text.
Following a recent update, predictive text now makes suggestions in ALL the languages, regardless of which language I'm typing in. Obviously that now renders predictive text obsolete for me because if I'm typing in English and the suggestions coming up are mainly foreign, that's zero use to me!!!!