I'm an American who was recently granted a french divorce. How do you go about registering a french divorce in California? Also how would you go about collecting the child support?

Where you married in france, where does the other party live France?

Did the other party agree to this divorce.

Depending on a lot of those questions, it is "if" California will even reconise the divorce. And if both parties are living in California you will most likely have to re-file the custody and support agreement to be approved by California if you want California to reconsie and inforce it.