Generally you should probably focus on what language is most
important for you to learn. People will learn a new language either
as a hobby or more importantly, for business. If it's for business,
you're most likely going to succeed at learning the language
because it is part of your survival.
Since you're in school, you might be learning a secondary
language at school and it may not be what you really want to learn.
If that is the case, then you could learn Japanese as your
"secondary" language of choice. Secondary being your second, third,
or fourth language, whatever the case may be.
It comes down to the math of it. If you only have 2 hours a
day to study a language, you can't learn two languages at the same
time. If you choose to take away 2 hours from watching TV or
playing games, then you have 2 more hours which allows you to
either learn one language quickly, or two languages slowly.
French takes about 700 hours to learn, Japanese takes about
2,000. If you want to learn both, you should learn French first and
then learn Japanese unless Japanese is a part of your career choice
and French is not. Maybe this might help you figure it out, maybe
not. I do wish you the best!
