Being an au pair is much more than being a nanny. Au pairing is an exchange of language and culture. The au pair teaches the children she cares for her language and her culture. The au pair is immersed not only in the day to day life of the host family with whom she is living, but also in the culture of the foreign country. Because of this, she will be forced to speak the foreign language.
Becoming an au pair in a foreign country is an example of immersion style language learning. You will be surrounded by people who speak the language you wish to learn, and if you want to communicate with them, you will have to speak their language. Your foreign language skills will improve immensely because you will be speaking the language every day and learning the language as it is really spoken. Foreign language textbooks are use overly formal grammar and they often do not teach words that you would need for daily conversation. As an au pair, your daily interaction with your host family, the children, the friends you make, and the people you meet shopping will help you learn. After spending a year as an au pair, you will definitely be fluent.