I don't know if everybody already know this site, but anyway.

Those who translate manga know just too well that many sentences and expressions they see in manga aren't covered in your average Japanese language course.

So, the Japan Foundation Japanese-Language Institute, based in Kansai region, opened an e-learning site focused on manga (and anime), featuring several explanations on the meaning of some common structures and wordings you can come across in a manga.

If you are interested, the site is

http://anime-manga.jp/index.html