If I had to guess, I'd put it down to whereabouts in France you live. If you're in the south, it is probably more advantageous to learn Spanish or Italian as your first foreign language. Likewise German in the east/southeast.

Whereas Japan is really only focusing on English with something like 8 years of mandatory education now.

I'd also call into question the methodology used in determining competence in this study. I've found that French people will tend to have much better conversational English as the languages are quite similiar, whereas Japanese people can be quite shy and hesitant. If this study is conducted as a written test however, you could easily find the Japanese taking the lead.