Whether Polish or English is your first language, it's quite an achievement. Maybe it's because I read a lot. But I notice that a lot of natural English speakers speak poorly.
I'm with you on Oriental languages. Though to my ear Thai is worse than Chinese, which is a shame considering their fascinating culture.
Given the opportunity, I'd love to try Navajo and Quichua. They sound lovely and the cultures are amazing. But sticking to Europe, I'd go with Italian and Romanian.