It depends on a number of factors. Simply put, the changes between Japan and North America are going to be far, far larger than those between NA and the UK, for instance. I don't think most games are even modified for Commonwealth nations anymore, they just leave the US English in - there's nobody I know who cares enough for this to change what they buy, and it's not worth the company's time or money to go messing with it if it won't impact sales.

As for localization between countries, it takes a very firm grasp of both languages, an understanding of cultural similarities and differences, and probably a high level of general common sense as well. I have no clue how to get a job in it, but you might try trade magazines like Edge.