Essential to understand Japanese in order to know what you're doing in MH3?

[Lafiel]

well..

I guess you should atleast learn the 46 Katakana symbols, for they often are used to write english words in japanese.
That could be helpful as I expect Monster Hunter to be a game with many "english" words (or better "engrish" .. you need some experience to know which english word they really mean when transcribing something from katakana to roman letters).