I get that this is just a fluff story and because the whole premise is (I Can't Understand Her Language) it would be weird for MC to actually learn how to speak fluently, but with that said it would be nice if he had at least picked up some simple words by now. Things like "Thank you", "Sorry", "Look" etc. should have been covered by now. It's also kinda strange that it's the Demon girl that's starting to learn Japanese and not the other way round...
Though I do see a lot of people putting a lot of blame on the dumb MC and while it's perfectly justified the Demon girl is honestly equally to blame, she's done nothing so far to actively try to teach MC her language and is now going soft on him by learning his.
Other then that a fun read, doesn't require any brain power to process so you can just relax while scrolling through, the new character addition will hopefully stir things up a bit and make it even more fun.