@TheKleptomaniac
@SasazukaIchimashi
At first I thought the MC was just being stupid for not trying to learn Mary's language, but after that chapter with Neah , I was convinced he is just physically unable to learn that language of that world.
Whatever language Mary and the other people are talking in, seems to be a language that humans (from our world) are unable to pronounce or even completely hear (due to different sound frequencies?), remember how he had trouble understanding Neah's name to the point she had to simply it for him. Neah's real name was depicted not as characters or letters in the speech bubble, but as gibberish with some letters (the letters being what he is able to recognize as regular human sounds), which again points to the idea that the MC is unable to reproduce or even listen to that language, think of a a human trying to learn or reproduce dolphin sounds, it's just not physically possible.