Anyone got any ideas. I primarily speak English, but I know Japanese and French equally well but not as well as English, so they both become muddled when I try to speak either..
I think you need to train your brain to see French and Japanese as two different languages. After saying a sentence in French, immediately say it in Japanese also (and vice versa). This worked for me for Spanish and Italian. When talking to someone in Italian, I would repeat the Spanish equivalent of every sentence in my head immediately after saying it in Italian