It's correct English for anyone to use: "I don't like that swamp, there are flies there", is correct and natural. Merely saying "I don't like that swamp, there are flies" though perfectly comprehensible, sounds incomplete, and more like something a non native speaker would say.
Because in Russian, only one syllable is stressed. The other syllables have the vowel sound reduced, which is something that also happens in English to a lesser extent (like the "o" or "a" in political, or the "i" in rapid). In мухи, the у is stressed and according to IPA the и takes on a short "i" sound.
I added the article to make a natural sounding equivalent in English: "There are some flies." Not accepted.
The fact that two different words, there and there, are spelled the same is just a fact in English. "There are" indicates existence, and "There" indicates location. Both words are needed in the English sentence.
I'm reminded of this little sentence. "I will spring over the spring using a spring next spring." We English speakers like to conserve words. :-)