I'm not knocking foreign films entirely, but I've got to admit, sometimes it's harder to make a connection with the material when you've got to rely on subtitles. Ferrell is more than enough to handle when he's just messing around in English, how are we supposed to digest his antics via text? If he's going to pull this off, the dialogue is going to have to be top-notch; he can't rely on silly slapstick like he normally does.
Of course this is possible, but I find it highly unlikely. What's the last Ferrell film you've seen that's not only enjoyable to watch, but genuinely funny, quotably funny? I've got to go all the way back to 2004's Anchorman for that. Ever since, it's basically been dud after dud, save for one or two pieces that only sink in after multiple viewings. Zoolander, Elf and Anchorman are really the only films in which I can tolerate and really appreciate his style of comedy. (Yes, that means I'm not much of an Old School fan.)
However, I've got to acknowledge the fact that Ferrell is taking a risk with this one and sometimes big risks mean big rewards. Could the Will Ferrell subtitled comedy actually be uproarious? Sure and perhaps the pressure to deliver a quality script rather than just resort to physical silliness will work in the film's favor.
By the way, does Ferrell even speak Spanish? Unless he's brushed up on the language since Anchorman, I doubt it.