Just as a language it's not that hard, but since you probably want to use it to understand Japanese in games or anime or manga or whatever then it'll be a little more difficult since the Japanese like to use cultural references and wordplays a lot, the latter is probably one of their most used forms of comedy.
If you're just learning the language for the hell of it and don't plan on actually using it for anything Japanese then it's about as easy to learn as any other language, and if you put time into it then you could probably learn a fair bit of it over the summer
