Where can I access the US language file ?

In the file: src/main/resources/assets/mekanism/lang/en_US.lang

Why do you want access it? Is there a mistake somewhere?

Nah, for personnal usage, do I need to decompile it ? And what to use then ?

Wait nvm think I got it.

Lastly, do I need MDK or I can use normal mekanism ?

What are you trying to do? (from this will depends what you should use)

I want to rename the universal cables, as I changed the recipes for them, and gave them a new "usage" in my modpack, so I also want to rename them ^.^

So how do I exactly do it ?

Open the regular Mekanism .jar (you do NOT need MDK, it's intended for devs only) with an archive program (7zip, WinZip...).
Open the language file located at src/main/resources/assets/mekanism/lang/en_US.lang in a text editor (Notepad++ is good, bu you can use Notepad that comes with Windows...) and then change what you want.

I'm not sure which of Mekanism.jar or MekanismGenerators.jar is the good one, just try.

Kk thx