If you are writing a mod that will add language (therefore people with the mod will be able to choose this language), then all you need is to add your lang file as any other.
If you add lang files into this folder in your mod:
You will add those entries to existing lang default one (so you can add names for new items for example)
However If you add your lang file into minecraft directory, you will overwrite existing one (Like that YOURS will be used with higher priority than original one, when SAME NAME used - otherwise you will add another language), then the path will be:
The texts you want also include are in
However after removing of the mod, you will still have original Minecraft language files.
You can look at more in-depth tutorial for lang files by BedrockMiner: http://bedrockminer.jimdo.com/modding-tutorials/basic-modding-1-7/language-file/
If you dont want to compile your own mod for this, then all you need is to rewrite original MC file (and it is stored in same directory as above when you open minecraft's jar with winrar, except that you need to omit
PS: For example IndustrialCraft2 has Pirate language added, you can open it's JAR with WinRar and look at it where and how it is done. (But be aware that there is something like copyright and you just cannot grab their file and use it on your own project without asking the authors)