Communication is never a problem on the exchange visits. Whilst some may only be able to speak their own language, many participants on either side are able to speak the other language to a greater or lesser extent, so there is never any problem with communication. Whatever your level of language skill you will find a supportive atmosphere which will give you the opportunity to improve your grasp of French.
In general, if you speak good French you will be partnered with French people who may speak little if any English. However, if you are a hesitant communicator in French the French organisers will always try to arrange accommodation in families with people who are good English speakers.