A multilingual class is a class where the learners speak a variety of first languages. Multilingual classes can be compared to monolingual ones, where all the learners speak the same first language.

Example
Multilingual classes are typically found where learners have travelled from other countries to learn a language, e.g. summer schools.

In the classroom
In a multilingual class there can be much more use of the target language, because it will be the only common language between the learners, who will use it for their normal interactions both in and out of class. Learner behaviour and cultures can be very different, which may create problems but can be used as a focus of comparison and discussion.

Further links:

https://www.teachingenglish.org.uk/article/john-simpson-using-multilingual-approaches

https://www.teachingenglish.org.uk/article/every-learner-a-language-learner-every-teacher-a-language-teacher-live-webcast

https://www.teachingenglish.org.uk/article/benefits-being-bilingual