Everyone understands Castilian, and I doubt comrades would give you a hard time as long as you seem to be trying a little. If you know Castilian—and especially if you also know some French—it doesn’t take long to pick up the gist of a lot of what’s said, and reading is easier still. Bona sort, camarada.

I wish I had advice as far as parties, but my time there was long ago and closer to turístic than residencial.

iirc there’s Moviment Socialista (trots), the only ones I know that do something at all. Maybe PCOE?

e: Don’t worry that much about Catalan, you’ll learn gradually.