I know Rusyns of Serbia speak the language similar to Slovak.

All I wanted to say is that ethnic term Ukrainian is a new one appeared only 100 years ago or so. Ukrainians of Russians empire were labeled as Malorosy. Ukrainians of western Ukraine were part Austro-Hungary. The self-identification term was Rusyn. Rusyn was an eastern Slavs who was either Greek Catholic or Orthodox. In early 1915 Rusyns of western Ukraine adopted Ukrainian ethnic term. Some kept their old ethnic term Rusyn. Particularly those living abroad.