I am the facilitator of a meeting and know that some colleagues are more comfortable using English while others are more comfortable using French. How should I manage the choice of the language used for the meeting?

As a CERN contributor, you should use and promote the use of both English and French. In this case, you should tactfully propose the language that best facilitates communication. To the extent possible, all participants should be encouraged to express themselves in the working language in which they feel most comfortable.