It is indeed a dilemma. I've thought about it myself and there could very well be 4 languages involved : the mother's, father's, country's + the language spoken between the mother and father, which might still be different of their mother tongues. 4 languages is definitely too much for a child to cope with. 2 is ideal, but can already cause confusion and possibly make the child's other intellectual developments stay behind (could be maths, or social skills or curiosity...).
I've learned a lot of languages myself, but, though I started as a child too I tended to hate other languages than my mother tongue till the age of 16 ! After that I couldn't get enough or learning (but progressively lost interest in other things like maths and sciences, which had been dominant till then).
The capacity of a child to learn a language could also depend on their intelligence and sex. Some will be better at languages than others, or just have a better memory or clearer mind. Usually girls are better at languages (have you read Why Men Don't Listen & Women Can't Read Maps ).
It is difficult to decide for a child's education. Not only the language is important, but the school (which is doomed anyway, as it's impossible to find a perfect school where all teachers would be marvellous and most other children friendly) and more importantly what you'll teach the child yourself. The most important is to encourage their curiosity and give them all the means necessary to satisfy it (that is much easier now with the Internet and educational CD-ROM's).
Don't force a child to learn something they don't like, for they could be disgusted forever.
Practically, do you intend to speak English and Japanese to your children ? This is probably ok as they are very different and thus less likely to be confused. You should speak only your mother tongue with the child and your partner only her/his. The child will instincively know which language to speak to whom and won't mix them up. What is the 3rd possible language ?