Certificate of residence according to double taxation treaty?
I am registered as an auto-entrepreneur in Paris. I have a client which is a quasi-state organisation in Hungary. They are requesting that I get a certificate from the French tax authority stating that
The Hungarian client says this is a relatively common certificate and should not be a problem to get. I am not so sure given my experience with French bureaucracy. I have a sample of such a certificate issued in Italy, in Italian and English. The key thing it must say is that I am resident in France according to article X of the double Taxation treaty between France and Hungary (they claim there is one). The purpose is so that the Hungarian government does not deduct taxes from my consulting fee.
Can you tell me first of all which office I would contact to ask for such a certificate? Do you expect it would be easy to obtain? What would this certificate or the concept of double taxation be called in French?