I have a UK password my wife is Spanish. We live in Spain from Oct-Mar. Once I have stayed more than 90 days, I travel with my wife and our libro de familia to prove we are married. In Spain that is fine. But as the libro de familia is in Spanish, does anyone know if French authorities are ok with it. I know it is an EU rule that I can travel within the Schengen zone (after 90 days) providing I'm accompanied by my wife. I'm just asking about the practicalities of using a Spanish document in France as we would like to drive from Spain to UK via France.

You better change your UK password for UK passport, or French border guards will have fun time with you.

Just get your ES marriage cert translated and notarized in FR. Although ES marriage cert is good in FR, not everyone will understand it.
Not sure if ES issue documents in international format under Vienna convention rues.
You do realize serious tax implications with your long stays in ES, if you dont plan it right.

Thanks.
Yes, we definitely want to avoid becoming ES tax resident.

but after 90 days in Spain, you SHOULD (by law) register as resident

True, but that doesn't necessarily make you tax resident.

agreed