The flowers will learn the language before Ren does.
@jamesglordduhs Because it fights against the most fundamental level of common sense the dude makes zero effort to learn the language, even tough he really likes Mary and doesn't mind living in that world. Sometimes when people move to another country, they might not put much effort into learning the local language if English will allow them to live more or less normally, especially if they are only planning to spend a few years in the country and then move elsewhere. However, Ren here can't use any language to communicate, plus back when he though he was transported back to Japan, he was quite distraught and wanted to return to the demon queen's castle. So, there's no logic whatsoever in his refusal to even try. Some people, myself included, simply are annoyed by such a thing.