I'm not a student of the Hochschule RheinMain but I would like to attend a language course. Is that allowed?

Sure! If the course takes place during the semester you can register as a guest-Student (auditor) (hier klicken) . The fee is 100€ (to be paid to the Student Information Office. You also need a signature (of the language instructor or of the Language Centre office), stating you are allowed to participate. 

I'm a student at another university. What do I have to do in order to attend your language courses?

You have to contact the student information office and will receive the form for student auditors ("Zweithörer") by mail or in person. Contact details: Tel. 0611/9495-1560, studsekwirz.fh-wiesbadende