"Je parle francais" reasonably well, and I have been many times in Paris. And I love trains.
Having said that, navigating the system from CDG to Montparnasse is quite challenging. It has its logic, but you need to know it. The unavoidably transfer to Metro is complicated, especially in the rush hours : a maze of corridors, lots of hurried people, and you are stressed after a long travel, migration, etc.
So, if you have the time, I recommend to choose the bus.
If you are time pressed, your budget is an issue, or want to travel as Parisians do, go to the train.
Two advices, for this case: remember to keep your ticket, because you will need to transfer, and there are inspectors, and fines.
The second comes from experience. Some metro stations (as Denfert Rocherau) have tricky door turnstiles. They open just a second to permit a person go, then close with an audible "thud", to prevent the entry of cheaters. If possible, look for a luggage roller slot, or the point where a station agent can open a gate, so you can go with your backpack. If this is not the case, keep your backpack
in front of you, not behind. Otherwise, you risk being trapped, with your backpack in the other side of the ¡"#
! door turnstile. This makes later for a very good anecdote, but at the moment is not so fun...I discovered later that "Denfert" came from the place where it is, formerly called "the barrier of hell". And rightly so, in my opinion.