It's a good idea to learn a language for each purpose, but I wouldn't worry about it all that much yet. Just focus on at least getting a language under your belt before taking on more. I've been using PHP for over a year before I went over to another language like Java.
Ludum Dare is next weekend if you want a little challenge at making a game in 48 hours which then gets scored by the community. :p For the preceding ones I've been using Game Maker and this will be my first time using Java!
You could even use this chance to make a game in HTML5 if you want to try learning it, it's a wise-choice and I'd totally go for it! Don't think there's things you need to know before making a game, I got into Java by skipping right into game development.
Just follow a tutorial or two