Well, Lithuanian is also a Proto-Indo-European language. But the one that somehow got sucked into a time warp from the past. And it even has a tonal pitch accent in addition to the stress pattern, just to make it more interesting.
The language seems to have more archaic features and forms than, say, closely related slavic languages, and its vocabulary has more similarities to old sanscrit than one's average european language.
For curios language learners this means that the grammar is harder than even (already hard enough) slavic grammars.