I can help! We selected an au pair three years ago whose English was so so bad she didn't even understand when we asked her if she wanted to be our au pair (btw, she ended up being awesome and staying with us for two year).
I did a lot of hunting around and found this company that matches English tutors with learners for Skype lessons. You can match with anyone you want to. I think she did 6 or 8 lessons, and while her English was still weak when she came, it had improved some and it really helped with her confidence.
https://www.verbalplanet.com/English-tutors.asp