Interesting! A lot of Basque are familiar with English, but it will be difficult at first unless you have some basic knowledge of Castilian Spanish.

What are you planning on doing when you arrive and encounter the Euskara, the language of the Basque speakers? You may want to check to see if the law regarding speaking Euskara in order to get a job is being enforced.

If you're planning on teaching English, there are plenty of different little private "schools" that are hiring now for the new year. I would recommend looking around in different smaller towns as in Donostia it's a bit more complicated (though not impossible) to find a teaching job. They'll want you to know Spanish so you should work on that, but I have seen people get jobs with very little knowledge of the language.

If you are wanting to work in public schools it is far more complicated, something you might research further once you are here for a while. That said, if you don't speak Spanish and/or Basque, that option is out for now.

And finally, if you want to work in any other sort of job, you'll definitely need to know at least Spanish.

Hope that helps a little.