Spanish. However, I'm bad at focusing at one language at the time so sometimes I flip-flop between Spanish and French. They are both Romance languages, so I can understand a lot (Spanish even more so since it's very similar to Portuguese (my native language)).
I have a little strategy for reading theory books. I keep two translations of a text in different tabs. Whenever I start to get bored of reading, I switch to the Spanish translation. When I start to get bored again, I then switch to English. Perks of being bilingual lol.