From my experience it is much worse than it used to be 5 years ago. I have been writing English, French, and to a lesser extent German on an iPhone since ~2008. Initially, the dumb autocorrect would just correct to the closer word in the dictionary corresponding to the current keyboard, but over time it would pick up more and more words I used regularly. At some point around 2018 or so, it was nearly flawless. I think it changed the dictionary depending on the language or the sentence, because I had different suggestions for the same mistyped word in the same document. Also, I assume that by then my personal dictionary was quite extensive.

And then they bragged about a new machine-learning improved keyboard and it went downhill. First, all keyboards became monolingual, which was a 10-years regression. And even in that language, it was very flakey. They added multi-language keyboards somewhat recently and it got slightly better, except that for some reason it changes the keyboard back to the English-only one regularly for no reason I can see.

It is maddening. For a couple of years it was fantastic.