Last weekend one of the choirs I sing in presented a concert which had been delayed and disrupted by COVID and reduced in numbers by choristers travelling. Alongside works in English, liturgical Greek and Latin, we sang works in Church Slavonic (a movement from Tchaikovsky’s Liturgy of St John Chrysostom) and Latvian (a new work by a local composer of Latvian birth or heritage).
Church Slavonic and Latvian are both Indo-European languages, so I was on the lookout for any words which are obviously related to other IE languages I know about. But the only words I could discern are loan words into those languages just as into English: kheruvímy (cherubim) in the former and fenikss (phoenix) and oranži (orange) in the latter (all heavily influenced by the pronunciation and spelling of those languages). There is also trisvętúju in the former, which is guessable as trinity.
Even though all these languages are Indo-European, they are obviously very different. Even though Church Slavonic and Latvian are both Balto-Slavic, they are obviously very different. Among other things, Church Slavonic is Slavic and Latvian is Baltic. Also, the texts we sang are liturgical dating to perhaps the 9th century and a 19th century secular poem.
Linguists started by comparing closely related languages, such as Church Slavonic, Bulgarian and Macedonian, and Latvian and Lithuanian, then work their way back from there, eventually linking Polish, Czech and Slovak, the Balkan languages, the Russian-related languages and others into Slavic and thence with Latvian and Lithuanian into Balto-Slavic and then Indo-European. (Some people have attempted to reconstruct further back than than that, but their efforts are speculative and inconclusive at best.)
PS The Latvian poem is Putns ar uguns spārniem (which I can’t find anywhere online) by Aspazija. The title translates as Bird with wings of fire. I wondered if putns is related to a certain Russian surname, but no, the certain Russian surname apparently comes from put (path or way) + in (belonging to) and probably means something like ‘one who travels on a path’. (I couldn’t find any authoritative source and am relying on several user-submitted websites.)