When it comes to choosing the Au Pair's nationality, you must take into account the following two factors:
Legislation of your country regarding the Au Pair programme, especially with respect to visas, and any agreement executed between your country and other countries as regards the Working Holiday visa.
Goal of the Au Pair programme for your family. Whether the goal is that your children practise a language or that you get help with the care of your children, the Au Pair's nationality will be very significant. In the case of families whose goal is the language, we recommend that you don't look for only Anglo-Saxon nationalities, since there may be other countries whose citizens have a perfect English level, even if it is not their mother tongue.