It's a standard feature of many mobile keyboards. I personally use MS SwitfKey, but GBoard has it as well, and I'm sure others have it too. You get to select multiple languages for which they'll do auto-correct, predictive suggestions, and swipe-typing.
It works surprisingly well, but it does sometimes fail pretty spectacularly when you're sloppy and mis-type too many letters. It also can't help much when words are ambiguous (is that an English "in" or a mistyped Romanian "în"?). Still, it's generally a godsend for typing in mixed languages (say, like in Spanglish, where you'll write unas palabras en Ingles, y altras in Spanish). They even include things like names of local politicians or cities or brands.
Windows does seem to have this as well as an option ("Multilingual text suggestions"), but individual apps might not (e.g. I haven't seen it in Outlook).