Although it is true that in some cases it's the student's individual problem to follow the course, some universities ask students to write a motivation letter in Spanish and sometimes they also make a little oral interview (it used to be by telephone, but maybe now they do it through Skype), so their general capabilities (Spanish included) can be evaluated.
As far as I know, the Universidad Nacional (UNAM) asks foreign students to make a Spanish test among the other exams they use to evaluate students' skills, to be sure they can follow the courses.
Still, I think that having an official certification would simplify a lot all these procedures.
