thanks again to all for your thoughts.
This au pair did not pick up French in a year - she worked for a French family in Slovakia for two years as an au pair/sitter before going to Paris. So I'm not sure how much that indicates about her ability to learn English quickly, though when I interviewed her she was very clear that her main purpose in coming here was to learn English (as opposed to, say, earn money, escape weirdo boyfriend, etc. - as seemed to be the case with many others we spoke with). She is a teacher by training and seems like a studious type (I know, you never know!) so I have to hope she'd be serious about improving.
I agree this may be very hard work and think we may ask her to come in and do a "day out" with me and my son to see how well she copes, linguistically, as suggested. I'm not really in a position to review things two months after she starts, since at that point I'll be dealing with a brand-new baby (or about to have one) and a jealous 3 1/2 year old who needs to be shuttled back and forth to school every day, and I don't see myself having time to locate new help, plus it would be pretty disruptive for my son to have the arrangement end right as the baby arrives.
Also thought about sending her to an "intensive" (15-20 hrs/week) language course for 2 weeks right when she starts, and just paying for it ourselves but not paying her salary those two weeks (cost of the course is several times her weekly salary). Unfortunately all the cheaper English classes around here only seem to meet for 2 hours once a week, and I can't see that really getting her very far.
I wish we could find someone who met all our requirements perfectly, but honestly we've been looking for months and had numerous false starts and two major let-downs (one of which was the au pair not showing up on the day she was meant to start, despite having only 2 days before confirmed to her previous family that she intended to come and work for us!). The thing most of these had in common was that the girl was not here yet or we had not met her, and it seemed like it was then all too easy for her to change her mind/find a different family, or just disappear if we didn't make her an offer fast enough. So I am very reluctant to make another offer to someone who is not here, and prepared to meet us first. That definitely limits the pool of candidates.
We just moved to the city we live in about a week and a half ago, so I don't have a strong local network of mums/nannies that I could tap into for suggestions (thank goodness for mumsnet!), and as I'm still working myself at the moment I have to limit the amount of time I spend on the hunt...especially since I've already spent an awful lot of time on it. We currently have a temp part-time nanny through an agency, to take us through till I go on maternity leave, but the cost is so high that I can't see continuing this once I am no longer being paid (said nanny, who is great, is also going back to school and not available after end of Sept.). Am also considering just getting in a local student to help with mornings/school runs, but it seemed like with the baby, a live-in au pair might simplify things (we have no helpful relatives or anything like that, and no friends here as we just moved...) and also provide some continuity for my son.
Hmm.