If you are teaching a specific language to people with different language backgrounds, which is often the case for ELL and ESL teachers, you can still put them in the same classroom. The process should be the same even if everyone speaks, for example, English but each student is learning a different language.
It is important to note that each student will be taking a different course (let's say English from Spanish and English from Chinese), and since each of them is different, the words and lessons will vary a bit. This means that some Duolingo for Schools functions and features will not be accessible to a multilingual classroom, such as skill assignments and vocabulary/curriculum preview. You will still be able to create XP assignments.
If you really want all features for each student, a workaround is grouping each language learner into their own classroom depending on their “origin” language, but then you may end up with several classrooms to manage.
How to create a classroom for students of various languages
1. Open the classroom on Duolingo for Schools and select "Classroom settings"
2. Click on the dropdown menu "Duolingo Language Course..."
3. Select "No specific language"
4. Remember to save!
Your multilingual class is ready!