I think we whall translate “onion adress” with “adresse onion”, since this is the “TLD”. I mean, it can be considered as a proper noun.
So far it was translated to “adresse oignon” (Glossaire | Projet Tor | Assistance)
wich bring confusion. People might want to browse .oignon adresses, and that won’t work.
also applies for “onion services” and “onion circuits” wich could be “services onion” and “circuits onion”
BTW, in the source, “Onion Browser” is translated by “Navigateur Onion” in french (OnionBrowser/Localizable.strings at 2.X · OnionBrowser/OnionBrowser · GitHub)