That's... not how sound works. Unless his hearing is on a new frequency that can't hear human speech, he'd still be hearing spoken words. Also, if he has a tongue, he should be able to practice to form words he used to use as a human, at least simple ones. Unless there's some weird magic involved, this "language barrier" seems awfully contrived.
Edit: OK, a later chapter clarifies that it is an actual language issue because they're from a different country. Still, the point about human speech stands.