I have English (US) and Tamil keyboards installed. And most of the time, my predictive text is in Tamil. It's very annoying. I'm also looking for a solution. I even I reset keyboard but still same.
I tried this steps and still same issue Settings > Apps > tap the three dots > tap Show system apps > tap the Samsung keyboard > tap Storage > tap Clear data.
Same problem here on my Tab S6, predictive text does not match the keyboard language setting.
I'm really disappointed (but not at all surprised) that Samsung Electronics ignores it's customers.