Acceptable in Bilbao: is it better to speak Spanish or English?

I was there last year. The locals do speak Euskarian and Spanish. There is a clear bias to Euskarian similar to how Canadians in Quebec or Montreal speak English: They all can do it quite well, but they will speak French by default.

English is touch and go: you can certainly do simple transaction like ordering food, buying stuff, etc but we didn't meet a lot of people that could engage in a deep conversion.

I'd recommend: If you are proficient in Spanish, use that. Otherwise, use English as it sets a clear context and some mishaps or miscommunication are expected and perfectly okay.

Great town & people to visit! Have fun !

EDIT: one tip: around 6pm or so most bars start serving Pintxos which is small sandwhiches and finger foods. It's very good (and affordable) and very popular, so the places are packed. In Spanish it would be called pinchos or maybe tapas, but it's always Pintxos in Bilbao

I have been four times to the Basque Country (and twice specifically to Bilbao) and I never had any problem speaking Spanish to Basque people.

Bilbao locals are usually native to both languages and sometimes they simply use a few words from one language when speaking the other one. For instance, some Basque words that I got used to hear because they are often used among Spanish dialogues are: "aita" (dad), "ama" (mom), "agur" (bye), "ez" (no), "bai" (yes), "zorionak" (congrats), "ongi etorri" (welcome) and "eskerrik asko" (thanks).

So, if you can speak fluent Spanish, just use it.

Regarding the "don'ts":

• Don't speak about politics (this is extensible to any country in the whole world).
• Don't miss the opportunity to have some "pintxos" and drink some "zuritos".