Congrats on your offers ! I think that if you choose to go to Waterloo there is still a lot of potential for you to discover avenues in the federal public service. I went there during my undergrad and I am currently working in Feds due to a previous co-op I did in my 3rd year. But of course, there are also lots of advantages when it comes to Carleton given that it is in the Nations capital and many federal departments hire master students... either way, not being able to speak french is not a big deal, I know a lot of people who are not fluent in french - and plus, as an employers - depending on the department may require you to take some courses in French.
In my opinion, french is necessary to climb up the ladder in i.e. being a Director etc but this is just my observation as a student who did a couple co-ops in the public sector.
Overall, if you are worried about French, you could always take some courses. Waterloo has a french course that is no credit in the MPS program, and I am sure you can take some french courses at Carleton, although I am no sure if the courses are an additional cost.
I don't know if this helped.. but best of luck to you on your final decision.