Umm, if you get a basic knowledge of katakana and hiragana (the two japanese writing systems that aren't ultra-complex), you can understand quite a bit.
Add that with the fact you at least sort of know what's going on, and I'd say you could comprehend a decent amount of it.
Of course, I'm in my third year of learning Japanese at school and I only know about 20 kanji characters and a textbook or so of vocabulary, so I'm still left a little clueless as to what happened.
I did download a copy of JPY 3 (don't ask
) last year and played it a bit when I got bored.The games are easy to comprehend, since home run is "homu ran" and strike is "sutoraiku" (also, pawapuro means Power Pro), but oncre I got to success mode, i had to guess at a lot of the things going on, though after going through each menu I found essentially where everything was (it was pretty limited, just practice, date (or something like that), skills, and options).