Suggested fallback

I'd think that es would be the appropriate fallback for this since virtually every Catalan speaker is also fluent and native in Spanish.

Except those Catalans living in France (Roussillon area) that almost never speak Spanish in their daily life... Not all those in Andorra do not speak Spanish (though many are trilingual and also understand Spanish and French, and frequently English but only as a last fallback).

But you're right, Spanish could be a primary fallback (not sure that this really help all those living in France and Andorra), followed by French as a secondary fallback, before the default fallback to English. It would be better than fallbacking always from Catalan to English.