Installation:
Replace the file "gta alien city folder/text/american.gxt" with this american.gxt

[10\12\2020]: Just remembered about this project of mine and decided to stop by and explain some things. I'm a native Russian speaker, but this mod was made in my early "learning English" days, so expect a lot of grammar mistakes. Blueby did a good job at correcting them (Edited version) but sometimes he got the context wrong... Sooo, yeah... Also, I see some comments saying that not all the mission were translated, but I'm pretty sure that they were in final version, at least I don't remember any Russian text left in the text file. And for the record, I never actually played Alien City mod, I just saw people wanting English translation and jumped on my horse, running to save the day. 
If I made this translation today, man...
Have a nice day, random person reading this.