How to change the Half-Life 1 language FROM FILES?

How does Half-Life work as a multi-language game? When I switch the language from the game properties in Steam, it makes an additional valve_{language} folder so the game does not need to replace the default asset files.

But I could not figure out how the game(hl.exe) changes the language. I mean, if I just copy the valve_russian into the root folder, obviously it will not make the game to launch in Russian. I could not find the exact file(s) responsible for that. The game preserves the selected language even when I launch it from hl.exe, which eliminates the possible role of launch commands in this process. After tracking all the files with git, I saw no other files change besides the files in the valve_{language} folder when I switch the language from Steam.

Then how does the game know which language pack to load upon launch? What should I change to make Half-Life read my custom language pack(without replacing the original assets and creating a mod)?