Hi!
I'm currently doing two languages at Advanced Higher (one of them being French) after getting A's in both for Nat 5 and Highers, so might be able to help a bit on this! 🙂
I'd strongly recommend doing Higher after Nat 5's instead of going straight to Adv Higher, the gap between higher languages and AH languages is quite severe, let alone Nat 5 to AH.
There's so much information that's taught in the Higher courses that's necessary to do well in Adv Higher.
In my Nat 5's I think I got 86% in French and 97.5% in German, but if I did Adv Higher in either language straight from Nat 5, I would've failed 100%.
But if you still want to do it, 100% speak to the french teacher who would be teaching you Adv Higher!! They'd be the best one to assess whether you're ready for the course and offer advice. 🙂
One way to kind of gauge if you're ready to do AH French could be to do some of the Higher listening and reading past papers timed and using a paper dictionary (for the reading) and then mark them afterwards, try to achieve 70% or more.
One thing to note is that Adv Higher languages are quite time consuming,
Depending on the other subjects you're planning on taking, you'll have to think about how much free time you can spend studying (especially without being becoming burnt out), and the extra revision you'd need to be able to catch up through not having done Higher.
Overall, if you're planning on doing French and Economics, it'd make a lot more sense to do Higher instead, a lot of Scottish Universities focus more on Highers/don't have requirements for Advanced Highers for language degrees (I have no clue how this applies for Economics degrees though sorry), so if you did Higher French and got a really good score, you'd probably have a much higher (sorry for the pun) chance of being accepted! 🙂
Hope this helps and isn't too negative!! ☹️