Modifying the language files allows you to change vanilla text in places that other methods cannot reach. One great example of use case for languages would be your own server logo in the pause menu or changing what text the disconnect buttons display.
Each language has it’s own file, with the languages code as its name. For example the lang file for United States English is called en_us.json. This means, that if you wanted to support every possible language a player can choose, you would have to override every single language. On the other hand, if your server only officially supports the English language, you would only have to override every English language variation.
Language files are always located in PACK/assets/minecraft/lang. (If these directories are missing, you can create them)
The way you override a language, is by creating a lang file and naming it with the name of the language you’re wanting to override. On this Minecraft wiki page you can see all possible languages. From there, you look for a language and use the the “in-game” locale code in the file name.
<aside>
💡 For example US English would be: en_us.json. See all the default values of en_us here.
</aside>