on the right of the keyboard: ù
but that's enough to want accents and symbols on the number row by default (&é"'(-è_çà) and numbers when pressing shift.
I think that's the reason that bépo (a French variant of dvorak which allows easy access of both common accentuated keys and numbers) is more popular among French speakers than dvorak is for English speakers, proportionally.
I have never seen anyone use an Alt+number to get these, I personally default to EN layout and switch only when I write in local language.