From a review:
“Villazon’s French is not exactly that of a native, but it’s more open and natural than that of most Spanish speaking tenors.”
There are two things. There is pronouncing it and there is singing it. Dawn Upshaw pronounces it fine, but pinches off her tone every now and then in an effort to get the sounds exactly right. Edith Piaf never pinches off her tone. Ever.
Sometimes when you see photographs of recording sessions, there is a language coach sitting there with the singers. In the conflict between tone and correct pronunciation, tone should win. There is a school of vocal technique that bases its methods on vowel modification. So having someone there correcting your vowels could actually throw the whole thing off. So where is the vocal coach who is correcting the correcting of the language coach?
Singing in French for non-natives is a tight rope. Mumbling is a popular tactic. I won’t name names. If they can’t tell what you’re saying, you can get away with a lot more.