Perhaps also from French. The Russian royal family spoke French and napoleon spent a bit of time in Poland, as well as guys like Mickiewicz and Chopin spending a lot of time in France.
Yes, French was another abundant source of borrowings, especially in the 18th and 19th c., when it was spoken on everyday basis in most aristocratic and noble homes.
As for Polish not having much contact with English until 19th C, what about the large Scopts migrations in 16th-17th-18th centuries? Granted, many would have spoken Scots, but also a fair few would have spoken English.
Still I struggle trying to remember any word that might have been a borrowing from either English or Scots inserted into Polish by these immigrants... apart from the word "kilt" :) Well, there must have been some. The thing might have been that the Scots assimilated very well, they never formed any kind of ghetto or diaspora so they probably just learnt Polish and mixed with the rest of the population. It's an interesting issue, actually, I'd like to read more about it. Did he Scots bring their families to Poland with them? What I know about it comes mostly from Sienkiewicz's books - there was a Scottish regiment at the court of Prince Radziwiłł, and there was the handsome Ketling, rivalling with Wołodyjowski for Krzysia's hand. Shame that I don't know much more.
There were many ethnic groups in Poland in these days, we had Scots, Armenian merchants, Ruthenians, Hungarians, Vallachians, Germans, Dutch groups, even Turks and Tatars - a true multi-kulti, each group with their language and insertions into Polish. And Polish is very good with assimilating borrowed words, they soon acquire various endings, prefixes and suffixes and get assigned their proper place in the tables of declinations and conjugations :) So, all in all, I'm not particularly worried about the future of the Polish language, it will survive and manage very well.