The teaching language for your course is set in the course description which functions as something of a contract with students – the course should be taught in the language outlined on the course description. If you would like to change the language of a course, you should contact your section director, but also be aware that there are regulations covering the language of teaching (see here).
Remember that students who may appear to have a high level of skill in a second language may still find it quite challenging to follow an academic course in that language. Providing them with a glossary for technical terms can be helpful, and it can also be useful to try to avoid using multiple different terms for the same concept or phenomenon.
The EPFL Language Centre offers French and English courses specifically designed to help lecturers teach in a second language (French or English). More specifically, if English is a second language for you, you can find more information here. If French is a second language for you, information can be found here.