There's no administrative interface for doing this, but it's very easy to do if you have host or superuser access to the website.
First off, let's make sure we're talking about the same thing. In DNN, Language Packs can be installed that will support multiple languages for the platform interface elements that the platform delivers such as login, logout, user profile, user messaging, administration labels, etc. Do you see how that's different from whatever content you might have presented on a page - that's content localization.
So, you can uninstall DNN language packs, but if you do, your site will crash with type 500 errors. The reason is that there are files in the /App_GlobalResources folder that also need to be manually deleted. Once you delete the relevant language files in that folder, your site will work again.
Next, delete the relevant language entries in the Languages table in the database.
And for good measure, do a search for all resx files in the App_LocalResources folders and delete them. You can perform a wildcard search such as "RU.resx" to find all Russian localization files.
- Uninstall language packs
- Remove relevant language files from ~/App_GlobalResources
- Delete entries from the Languages database table
- Do a wildcard search for resx files and delete them
- While the above step will restart the web server, restart the application pool anyway