When we had au pairs, they came through an agency and were required, under their terms and conditions, to attend language school while here. They were obviously here to learn English and we were required, as part of our terms, to help them with that and speak English to them.
Other duties were also agreed in advance - mostly childcare, but some household cleaning (to do with caring for children) and these had to be laid out in the contract with the agency too. (As it happens, in our case, our childcare needs were specific but not overly time-consuming, so we NEEDED AP in mornings for an hour, most of them collected from afterschool club a few days a week, and there were a lot of extra hours that they had either completely free or that we asked them to do a certain amount of housework but they were free to decide on timing - so one morning a week, or one small job a day, to suit themselves).
If you were clear in advance about wanting them to speak their native language to your children, that is different. In most cases, AP's do come to improve their English as part of the experience.
So IF you were clear in advance, and the AP finds it hard to fulfill that part, you need to talk again. I like the idea a previous poster had of giving a specific thing to do, as part of the routine, that AP will do "en francais" rather than leaving it loose and up to her to include. So bedtime story, or perhaps the daily up/dressing/washing teeth etc in the mornings, or over a specific meal (and only 1 meal a day), or a specific time to do some games/dancing/singing in her language.
But make it clear - and DO work on her English in a supportive way with her too, so make sure you have a time when she can sit with you and have conversations/debates etc that you are helping her. Preferably when the DCs aren't around so you can talk about adult stuff (things like current affairs) or can get into a debate, and you can correct her without undermining her in front of the DCs too.