I agree 100%. Had the same experience with French as well as German. So true about German dialects. Most people do speak Hochdeutch, but our landlord in Munich did not. German TV used to put subtitles for some of the lugers because their dialect was so hard for normal Germans to understand.
I think the au pair experience can vary a lot. When my kids were young I’d take them to the local park and there were always lot nannies socializing with each other, but possibly within their ethnic groups. I’ve known people who had good experiences and felt they became more comfortable with the target language, but a lot will depend on the age of the children.