Before starting, consult this page[wiki.facepunch.com] for a more detailed explanation on this process.

You can find the english translation of the addon here[pastebin.com].

In the resource folder of Garry's Mod or the resource folder of an addon, make a folder called localization if there isn't one already, then make a folder with the code of the language (consult the website in the introduction). After that, make a file named bftui.properties in the folder you just created.

Fill the file with the contents of that file, then replace each string with an alternative in your language.

After that, your changes should display in-game if you changed your language from english. I recommend you send me the edited file for the language so it can be officially implemented, or you can publish it on the workshop if you so desire.