If English is your primary language whether you like it or not, you didn't travel so far to practice English, you have heard stories of locals being offended by arrogant tourists who walk into a joint and start babbling in English like everybody is supposed to undertsand, and you enjoy showing off your Spanish. None of these apply after you go through security in the U.S. Embassy. Going through security is more Spanish than English but either will do. However, the consular staff speak English as their primary language and learned Spanish. You do not need to impress them, they do not need to learn Spanish from you and you do not need to learn Spanish from them. Speak to them in English. Pay the fee in pesos even if it is one percent more, just for fun.