It's definitely possible, so if this is something you want to do, then don't get your hopes down! I found this one girl's
blog and she works as an assistant manga artist in Tokyo right now, for the series Prince of Tennis. Apparently she didn't know any Japanese when she first got the job, but she's slowly learning! So it seems to me that you need good luck and good opportunities (eg. being in the right place at the right time) more than the ability to understand Japanese. Wish you luck!