The 3 times I have used that desk, they did speak English. And in 2 of the occasions it was after 21:30...Agreed that they are a bit pricey, but I remember there is a bit of discount if you buy a round trip so it may make sense.
I've used a late Roissybus once, and it's not an experience I will actively try to repeat. It was packed and luggage was flying all over the place.
The best VFM way to get from CDG to the centre, though, is the local bus going through the Citroen factory and Pte de la Chapelle. It's slower than the Roissybus but not too slow (50 mins to Pte de la Chapelle, another maybe 25 to the centre), and costs a fraction of the price (no more than 3 euros each way)! No Australian backpackers with 3 months' worth of luggage, either. Wouldn't do it at night tho.