Language skill requirements differ from country to country. While you will have an interview in English for the
US visa,
China as a rule doesn’t require you to pass the interview in Chinese. Some embassies might ask as well for an official certificate of your language knowledge. Once you shortlist the number of countries you would like to go for an
Au Pair program, contact their embassies in your country to find out the language requirements. You can also check
this page before contacting the embassy.