I did both Chinese and Japanese SL, however, I discontinued Chinese SL after finishing U1/2 as a Year 10 student, as I didn’t enjoy it, nor did well in it at all. Japanese SL I did all 4 units of and did relatively average in SS-wise (I’m not really a humanities or languages person), but had the better experience in due to genuine interest in the subject*. It did require a lot of effort and time though, but these classes were perhaps one of the most memorable of my high school classes due to the social environment (I had a bunch of friends who did this with me).
While scaling was present (Japanese gets quite a bit, from memory), I’d definitely not choose a subject purely for its scaling, but rather whether you like it and whether you need it at all for course beyond VCE (languages tend not to be prerequisites though). If you’re genuinely interested in French and Latin, then by all means do the double languages. If not, I’d reconsider your reasons for picking these.
* I also have a Japanese minor from uni.
Note: Was typing when Remy and you posted. Will leave this here regardless for the different perspective.