Well I lived in Spain for 8 years, and it never offended me if someone assumed (correctly obviously) that I spoke English, and it makes no difference to me what language I speak, as long as the other person feels comfortable using that language too. Now I'm back in Ireland, and I was working in a shop for a while, and if someone came in and I heard them speak Spanish, I would address them in Spanish. From my own personal experience, it is always nice to have someone talk in your own language when you're abroad. I've never had someone assume I spoke a language I don't, but I imagine that could be annoying, yes.