Although for English learners to study correct pronunciation and native accent is mandatory, living practice reveals a diversity of accents. Multilingual class gives students an opportunity to get to know various accents that will help them to be comfortable while communicating with people of different linguistic and cultural backgrounds.
If you learn English in a multilingual class it allows you to discuss English grammar from the perspectives of other students' languages. For instance, when you discuss English personal pronouns, you may ask your Arabic, French, Spanish or German-speaking mates to provide an equivalent for the English pronoun "you" in their own languages. The options will be multiple; to distinguish between them is a rather complicated task. So you will see the universal character of the English pronoun "you" and the simplicity of the English pronouns system as a big advantage for English learners.
Multilingualism of the class contributes to a psychological comfort and confidence of those students who might be behind the class. In a monolingual class, a student who is behind the others might feel uncomfortable. In a multilingual class, it is always possible to appeal to that fact that this particular grammatical or syntactic phenomenon of the English language is not represented in that student's native language, and, thus learning the concept is more challenging.
When you learn English in a multilingual class, it is easier for everybody to find new topics for conversation, because it is possible to discuss cultures, traditions, holidays, cuisines and landscapes of different nations. In a multilingual environment it is easier to stimulate discussions on such topics, because all of them seem non-trivial to the audience and allow to make little discoveries of other cultures.