I am not a constitutional expert, but I would lean towards believing that there is nothing coercive about the word “encourage”. The objective is to draw francophones' attention to the fact that speaking in their mother tongue can lighten the interpreters' burden, as it may be something they are unaware of. I want to remind everyone that, in general, about 75% of what is said is in English and 25% of it is in French. So there is already an imbalance between the two languages.
If we had to hold a debate on the quality of the two languages in Canada, I think we could do it on so many things other than simple encouragement.