There's also EU. You can study a lot of stuff in English here

So perhaps it's more like German influence. Maybe they calqued the German terms, I don't know how it was in Latvian.

… Both Latvian and Old Prussian had been under German cultural pressure, and thus it seemed justified for the revivalists to follow this way of expanding vocabulary, assuming that the mechanisms of borrowing these words in the areas penetrated by German language and culture were universal.”

Yea, this too ofc.

I found the quote. It's from “Language Practices in a Family of Prussian Language Revivalists” paper:

“Some terms connected with the development of the industrial revolution were created in in a way similar to how they evolved in the Latvian language. …

And speaking of Baltic and Slavic, Proto-Balto-Slavic is generally accepted. There's a huge overlap in basic vocabulary and initial sound changes are the same.
There are even articles proposing that West Baltic was the first to separate from the proto-language.

Maybe a better example would be Irish and Spanish? At least there's a hypothetical Proto-Italo-Celtic.

And what are the differences?
I know that they've created new vocabulary by modifying words from other Baltic languages with Prussian sound changes, and as far as I remember there are some borrowings from German for industrial stuff, but does it differ in terms of grammar or something else?

What does Kaliningrad represents here? Just another part of Greece?