First of all, this has no use in-game, but a forum moderator said it was ok to post this to the mods category.

What you can do with this folder:
- make mods and change multiple translations without having to painfully trace hash sources
- you can pick up a certain language and use it in your project

and ofc I can't decide what you're gonna do with this.

I've seen a lot of threads even outside of planetminecraft that they want a folder like this and have been looking for it.

When downloaded, you can open the ZIP file (MinecraftLanguages.zip), and please read README.txt first. After that, you will see 121 .json files upon opening the "lang" folder.
The language files are not formatted, so u could have good use of a json prettify plugin.

I will warn you though, these json files are very heavy in a non-compressed state.

How would you get the languages without this folder?

The process is very complicated. First, you will have to go to minecraft assets, then indexes and open the txt file of your version. Then you will find the language by the lang code (eg. en_us), and then copy the hash value. Then go back to assets, then open objects folder, find a folder matching the 2 first characters of the hash value, open that folder and find the folder with the original hash, and there you go.

Tip
If you happen to be using Notepad++ as a text editor, press the Plugins tab > JSTool > JSFormat, to format the language file.

As an example for what this folder can be used for, check my Block ID resource pack.

The languages were compiled in one folder by me using a python script I programmed myself.