Assuming you do not mean things like Ainu languages then you might want to clarify what you have heard. There are a variety of ways people categorise Japanese, and I will skip over the rudiments like kana vs kanji and as a start
In general you are likely to see contemporary modern Japanese using characters largely from the joyo lists, unless you are going for games aimed at younger people (so in games most will meet it when translating a shonen manga based game) in which case you might be restricted to a lesser list. Said lists get revised often enough and not every writer is going to keep up so don't stick religiously to the current one. In a lot of older games and no small number of new ones, presumably for reasons of space, a lot of the formality, even by modern standards, is dropped, probably not to the extent that speaking Japanese as you find it in games would be like turning up to a monocle sporting English country mansion gathering and speaking in gangster rap but you would probably not be confused for a native speaker.
There might be older kanji similar to how people use something from lists of interesting words in things to spice things up a bit*, and sometimes you might find a character speaks in a very old fashioned tone for comedic effect or because they want some effect (some find the Japanese taught in some American schools to be closer to this than not at times which I am told is endlessly amusing to some, and having some people learn English from Victorian novels and encounter something like it I can safely say it is). At the same time you might encounter slang so some would probably point you at http://dic.nicovideo.jp/
I think it was, both of which are kind of the Japanese equivalents of urban dictionary.
*writing systems also apply here so you might see things like tensho script which looks quite different to other font styles used in Japanese.
I don't know whether to point you at JLPT resources. JLPT stands for Japanese Language Proficiency Test and is fairly well liked by Japanese businesses and definitely universities as a means of gauging how well you understand Japanese. At the same time it is also somewhat artificial and test like, as most such things are for any language, so I definitely would not encourage doing that to the exclusion of more conventional/native style learning.