Well... it the depend of the person, their original language and the language they want to learn...
A colleague learned french in few months, she still need to.improve
He writing, but the reason is because she speaks Portuguese... while French has nor been a challenge, she is having problems with German!!!
While people who speaks English can learn German faster than someone who speaks Spanish... also Japanese have a lot of problem with English, but they can learn Spanish really fast!!!
Well, there are a lot of methods but yes, portuguese as french, italian, spanish are of latin root, while german, english and such comes from a germanic root. Japanese and so korean are in the altaic group of asian languages. I guess also it have to be with your own experiences and likes too. I love languages and still study new ones every time I can. Be it from any and everywhere.
I like the process of learning in this manga. It's probably doable for a lot of people to learn a new language in 3 months, but some mangas have their characters speak fluently in only a few weeks and I'm like dang lol. I think it's cute and it looks like a nice setup for other lovers in the story. I wouldn't mind extras of this couple too. Also a love story for the brother. I feel bad he's alone.