If you’re multilingual, go ahead and see if you have a common language. A lot of times, students have some basic English skills but are still working on their confidence. A common phrase you'll hear is: "I don't speak English" even from students who actually can speak a little. You can speak slowly and provide additional context with hand gestures to communicate. Drawing on a piece of paper with a sharpie or typing out a word in the chat can be helpful as well.
However, at the end of the day, Cambly is a place to practice conversational English, not teach English basics. So if the student has English abilities so low that he/she cannot hold even the most basic conversation, you have the option to very politely end the conversation. For example: "I'd love to help you, but you should come back when your English skills are higher." Then smile and wave goodbye.