If I may, I am going to answer in English, because it will be easier for me. I apologize. Sometimes, it is a bit difficult for me to manage in French on certain subjects, like this one.
The language of publication or language of application is somewhat reflective of the language of work in a discipline as well. I would like to hope, although I don't have evidence to this effect, that applicants who are francophone, if they choose to apply in English, it's reflective of the fact that their work environment, when it comes to communication of scientific knowledge, is more in English than in French. This may be the nature of certain disciplines of research.
To be honest, I don't have enough detailed data to be able to validate that. I can certainly say that when it comes to the natural sciences and engineering disciplines, the vast majority of research communication is done in English. It may be the case that this influences the choice of language of the application itself. Again, that is not based on analysis.