The host family is instructed to leave you enough free time so you can take language classes at a language school. Your working hours should be organized in such a way that you can take a language course.
In most countries as an au pair you don’t have to take language classes in a language school. But it can help you to take a language course in your host country so you can not only improve your speaking but also learn the grammar. In a language school you can also meet a lot of people so you won’t feel alone and homesick. Ask your host family what language school you should go to.