I don't have much advice as I don't know Spanish, but my old band did record an album in the south of France, so I've experienced the language barrier in a studio environment.
As others have said, the most important thing is to speak slowly, trust me on that one. Simplify certain sentences and try not to use complicated English (but don't be a condescending prick either). Show the guys a good time (when not working hard) as this may be somewhat of a "career highlight" for them.
Looking forward to hearing the end result!
I do have a few things planned for them actually. They will be here during the Woodward Dream Cruise which is a world famous annual classic car cruise here in Detroit which is literally two blocks from my house. I also plan on doing a fair amount of grilling to end a few of the days, and probably trying to have them come out to my gigs as well because I still play out during recording schedules (even though my body hates me for it!)
what was is like recording in France? I would love to go there.