[nj923f]

This hasn't been controversial in Latvia at all, at least in 90-00's (not that scientific theories should be judged by how controversial they are among laymen). The theory that I heard in several Latvian schools, is that lithuanians and latvians had a common language with slavs and they got separated somewhere in bronze age. Lithuanian language supposedly is the most archaic language alive (that is closest to proto-indo-european language by some measures). As far as I undestand there is consensus between linguists working in the field, that Baltic and Slavic languages are very close, the only questionable part is whether there was proto-balto-slavic language or whether these languages acquired similarities over time (also, some suggest that slavic languages separated from existing proto-baltic language).

Given tendency for revisionism and troubled past between Baltic states and Russia, one could see how it seems so alien to some people.

Btw, here is article on this topic: https://en.wikipedia.org/wiki/Proto-Balto-Slavic_language (also https://lv.wikipedia.org/wiki/Baltu-sl%C4%81vu_pirmvalodas_h... which is much shorter, but has different references), and it seems that theory itself actually has been proposed by western linguists.