I write in multiple languages other than English, and the keyboard support for all of the other ones is terrible on the iPhone, so I just do it all on the English keyboard. But it’s absolutely destroyed predictive text for me. The keyboards dictionary has learnt an incoherent mess of vocabulary, including typos that it didn’t know to correct, and random words that it always capitalizes because (I presume) the dictionary added the word after I used it at the beginning of a sentence and it now thinks it’s a proper noun.