Can we use "could/might" in the mixed conditional?
If I could speak French, [STRIKE]last night,[/STRIKE] I would have understood the French tourists.
If I had spoken French last night,[STRIKE] I would have understood[/STRIKE] the French tourists would have understood me.
See above.
It doesn't make sense to wish that you had an ability on just one evening. Either you can speak French or you can't. The situation won't have changed between last night and today.
If you can speak French now but you couldn't a year ago (because you've learnt the language in the interim), you could say "If I had been able to speak French last year, I would have been able to understand the French tourists I met at the local shop".
The second example you gave above suggests that you are able to speak French but you chose not to last night. If you had chosen to speak French, the tourists would have understood what you said. I would take it to mean that you spoke a different language to them.
If you don't speak French at all, it would make a lot more sense to say something like "I wish I could speak French. I probably could have had a wonderful chat with those French tourists last night".
