Can you give us some details about how the course is set up as a multi language course?
I suspect the course's name has the multilanguage tags embedded in it. The certificate code doesn't pass the string through the filters that would process it for screen display.... but I would have thought you'd see "<span ...." text in the name of the course.
The certificate printing code will get the right strings to use for the rest of the text on the certificate based on the current display language, but at least on our system we don't have a spanish version of the certificate.php file with strings. (There's a French version, but we're doing our own translation for Spanish.)