Well to start off, their language are very different from ours, english is latin based, as quite a few other languages, asian languages are not, the sentence structure and pronounciation are pretty different as opposed to let's say italian or french.
To compare to japanese for example, they don't pronounce some letters the same way we do in english, (Their U is pronounced like we do in french rather than english) the noun, verb and adjectives are also defined by another word that serves as a suffix to tell what the word was, but the sentence can pretty much be written in any order.
An infinite loop of MF DOOM muttering "aight then".
It's an American movie set in (I haven't seen it so correct me if I'm wrong) Japan. They're not gonna have everyone speaking Japanese in an American movie, so they just throw in "Asian" accents to make it more believable. It's the same reason everyone has a British-sounding accent in American movies set in Europe or Middle Earth or something.