I translated the add-on into Russian, how do I enable it? If I replace the en_us.mcfunction file, nothing happens.

This is not the right repository for WorldTool questions. I have responded to you on the Discord server.

Translating the plugin just means changing the few strings that the plugin contains (I assume you mean the biome plugin). Since you're using a plugin with a language that WorldTool doesn't support, you can replace the lang file like you did, then enable Reload in the language settings.