Help her to find other APs in your immediate area by mapping out where the HFs live, and let her take it from there. It's better for her English progress if they do not speak Polish. Plus there will be more opportunities to get together if her friends live near you. Likewise, connect her with English classes in your area; she'll meet local friends and improve her English there.

There is also a national Polish church in MD which is pretty social; our Polish AP met Polish friends (and her eventual husband) through the church. This was years ago, but I think it is the Catholic Church in Silver Spring.

When I was a new host mom I also did try to find a friend for my new shy au pair. Didn't end well. The girls who needed their host parents to find friends for them, turned out to have mental issues and soon went home.
Letr find friends herself and other au pairs who are not necessarily from her home country. This is why she came here.

Hello,

We live in Chevy Chase and just had a new au pair start with us a couple of weeks ago. She is from Poland and her English is good but still improving. She is such a sweetheart and I was hoping to help her find some Polish au pair friends that would help her transition. If anyone out there has a Polish au pair that might be up for a new friend, please DM me at nilofera@hotmail.com and we can put them in touch. Thanks!