As vivster said just "learning the Kanji" is not enough.
It's a good starting point to learn to differentiate and have a basic understanding of the main meaning of each of the ~2000 Jouyou Kanji and the ~600 Jinmeiyou Kanji (intricate games like many RPGs do use pretty much all of those), but that won't give you the ability to read the language, as most of the words are compositions of 2 or more Kanji and it's impossible to learn all the relevant meanings/readings by just looking at the individual Kanji.
Ofcouse knowing Hiragana/Katakana is a given, those are easily learnable within a few days/a week and some games "for kids" like Ni no Kuni have Furigana (Kanji readings in Hiragana above each Kanji) that makes them faaar easier to play in japanese.
