While Murray is able to translate just about everything Alexei is saying, it’s unfortunately not word for word. Basically, what the Russian scientist conveys is that there used to be all sorts of “keys” (a.k.a. openings) to the Upside Down in Russia but none of them would actually open. So, the Russians came to Hawkins because they knew that the key had already been opened once before. They’re in the process of trying to open it back up because it’s currently “healing” and that’s what they’re doing under the Starcourt Mall.