You really don't need to specify where in the EU, because there are vastly more people in France communicating regularly with people in other neighboring countries. If you need to specify where in the EU, then you need to specify where in the US as well, because France is about the size of a single US state.

Americans mostly talk to other Americans, who also have unlimited texting plans. French people are probably talking to people who live in Belgium, Germany, the UK, Spain, etc. who will all encourage them to use WhatsApp.