First, here's this:
http://www.playstationlifestyle.net/2012/02/05/how-to-play-japanese-games-and-rule-at-it/
Depends heavily on the game.
You might be able to get through the gameplay of a story-heavy game like Xenogears, but you're gonna miss the conversations and most plot details. That's a game available in English, but I just picked an example of a story-heavy game.
But Jacky's pretty much nailing it. I disagree on the percentages of course -- 99% seems a little high to me ^_^ -- but yeah, if you're a gamer, you know how to game and you can pull it off in any language.
In my work (I play and review these games a lot), I do, however, see a lot of people missing things within games because they don't understand enough Japanese to know otherwise. Some easy games will look misleadingly hard because the players don't know what they're doing, etc.
...But I'm getting sidetracked. On the whole, you don't need to be a master of Japanese to play and enjoy Japanese games, especially if story is no concern.