@Mixander Nobody is saying "wtf why isn't this guy 100% fluent in the language already?" People are amused that Ren still knows literally 0 words. He's also made no effort to teach Mary any Japanese. Even with no common language, it could be possible to pick up very simple, limited vocabulary for everyday objects and actions within a few months (over several years there's no excuse). For no apparent reason, this process is never shown to have been attempted or even shown to have been
considered by any of the characters. This wouldn't be too much of an issue if this manga had enough restraint to carry on as a pure gag manga. It would still be hack writing, but nobody reads this for its
brilliantly executed story; it's fluff. But no, by the looks of things, the mangaka wants to add a little plot and intrigue (the song stuff from last chapter, the story of the 1000 year old demon, etc). You add plot, expect your writing to be critiqued as other stories with plots are critiqued.
It's shown in Chapter 10 that in this other world it's understood that situations, ideas, and objects can be represented through drawings. Mary grasps the meaning of the "thumbs up" gesture by Chapter 9. So that means their "culture and common sense" isn't so radically different as to cut off those avenues of communication and learning. They're just inexplicably unexplored by these characters. (again, look at Heterogeneous Linguistics, where you're dealing with species that e.g. can only produce sounds in frequency ranges inaudible to humans or that cannot see representations in our drawings. THESE THINGS ARE AVAILABLE TO REN AND MARY)
Oh yeah, and before anyone says
muh cases: it's not a question of whether any approach would be of utility or would actually succeed. It's that nothing's even been tried. Ren isn't even shown to have
thought about this entire topic—which is bad writing, unless we're meant to see Ren as functionally mentally retarded. It's an extremely obvious point to consider.
