Hi Miguel,
Well THAT depends on who you ask ;-)...and where you are!
If my work load is any indication, then French is in the Top 3 with English & Spanish.
Here in the US, English is obviously #1, Spanish is #2 and French is #3
In Canada, since it's bilingual, I assume the breakdown is: English #1, French #2 and Spanish #3
Liz is right, it depends on where you are and who you ask.
The VO online marketplaces such as V123 and VOICES, could be a good way to measure it. I understand that the second most requested language by voice seekers in both marketplaces, is Spanish.