Never had a chance to do research there myself so can't be of too much direct help, but some very general advice. I can only assume that you're heading there for a reason - to work with someone or in a particular institution, so that's a logical starting point for building connections. More broadly, you can check university departments and staff pages for people with shared research interests ahead of time, as well as any relevant galleries and archives you plan on using, and get in touch with anyone you think might be worthwhile. It's often worth getting in touch with any institutions you want to visit in advance in Spain anyway, so you may as well take the opportunity to get a point of contact. If you want to immerse yourself then avoid the usual expat scene (I can only assume Seville has one...), but it may be worth checking if there are language exchange groups or similar, which often have locals keen to practice. If you're interested in the civil war in particular, it may also worth checking if there are any historical memory groups in the area, though these may vary wildly in terms of activity, organised-ness and how much they welcome the involvement of outsiders. If you do go down that route, don't be pushy about it and make sure you're engaging with their work and community on their terms. More broadly, if you have hobbies at home that you enjoy (or always wanted to try), see if there are any local groups that might welcome new members.

Depending on the kind of work you do though, it may still turn out to be quite isolating, so be prepared for it to take a while to find your groove and make friends, especially if you want to connect with locals rather than other visitors. There are a lot of friendly people to meet though, and foreign academics tend to get some respect and welcome, so you'll probably end up fine eventually.