As Mike Cash pointed out, you do not need to know any Japanese do teach English in Japan. You will need to know some English, but even that depends on the school. There are some jobs available here where you could little English and no Japanese and be perfectly competent at your job. But, that's not really here nor there...
The point is, if you want to move to Japan and teach English, don't worry about what your Japanese is like before you go over. It will improve when you get there. Or, if you don't feel like studying while in Japan, you can give up entirely and still have a successful fun experience as an English teacher. I took a summer intensive course the other day with a guy living here 20 years and can't write out his own address. Another guy living here 10 years, doesnt know all that much, and still pronounces things like a fresh off the boat American. And those are college professors.
I hope that after 2.5-3 years of living in Japan (and a tiny bit of self study before I came), I will begin approaching a low level of fluency. Right now, its been slightly more than a year, working as an ALT. Another year as an ALT, then maybe 6 months in a hardcore language school, and we'll see where I am. If I get to the point after 2.5 years, I will be quite happy. But then again, I work very hard.
(and when I say fluency, I don't mean native level)