The fact remains that 35% of your employees don't feel comfortable writing in the language of their choice.
Although I haven't done any scientific studies on this, I'll give you my opinion: I would have expected a candid answer to the effect that you feel comfortable writing in French because you know the message will be sent and there will be no problems at the end of the line.
However, that's not the case because sometimes you feel an obligation—from what I understand—to write in English to be sure it's understood at the other end of the tunnel.