Um, I think Spanish is the the language that offers the most utility and ease to the average American. Yes, French has massive influence on English, and it is probably the easiest language to gain vocabulary. However, I feel it helps mainly in the written form, as French also has liaison, which takes a while to get used to hearing. Spanish, on the other hand, does not have liaison, and the spelling of words actually matches the pronunciation, unlike French. Plus, there are Spanish speakers just about everywhere in the US.
She said it was her grandmother who was born in the US and learned Polish from her parents, so this was before World War 2.
If you guys never heard of mutual intelligibility: https://en.wikipedia.org/wiki/Mutual_intelligibility