You are the customer so do whatever you feel comfortable with. If you speak the language then by all means sit up front and have a chat. Otherwise, what is the point?

I doubt any driver would be massively offended by a back seat passenger given that you are (I'm guessing based on LAX) American and they will be very familiar with the layout of an American taxi based upon decades of American TV/movies.

Also - after a while you may note as you slide in the front seat, them invariably having to shift newspapers, bottles of water, small firearms etc off it. If they have stuff on that seat then they will be expecting you in the back.

Enjoy Barcelona - it's a cracking city with very good public transport - last time I was there I didn't even need to take a taxi...