Poland was not in the USSR. Polish remained the working language in the Polish People's Republic
> Hindi is probably another example of a language that the british empire tried to exterminate
Hindi-Urdu was never exterminated by the British. In fact, it was the British that helped make it the de facto language in most of South Asia.
Before the British, Dari was the working language of administration. When the British began co-opting local administrations in the 19th century, Hindi-Urdu was used as the primary register, and my family has ancestral documents showing this change (Dari/Farsi to Urdu/Hindi to English for land documents).
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The only dead language that I can think of that was revived was Hebrew, but modern Hebrew is entirely different from that which was spoken millennia ago, and is a mixture of litigurical Hebrew, Arabic (plenty of Mizrahi influence along with the Sabra movement during the start of Israel), Russian (heavily used for mechanical terms), and German (heavily used to scientific terms).