A course in a foreign country will help you to deepen and improve your language skills. It is awesome to be fluent in a language and that will raise your self-confidence. If you have the ability to express yourself, the communication with your host family will be uncomplicated.
Moreover, a language school is a good place to get to know new people, also other Au pairs. You shouldn´t forget that the integration in your new home town is very important.
You can finish the language school with a certificate, which will be added to your curriculum vitae. Especially universities will appreciate the proof that you´ve learned a language.
But who has to pay for the school?
That depends on the country! Usually, Au Pairs have to pay the coast for their language course. The host family can offer to pay for the additional costs as well as the application tuition for the class.