When you use the Microsoft Locale Builder tool to build a custom locale, it asks for a lot of fields.  Two may not be obvious:

The Two Letter ISO Language name is permitted to be 3 letters for locales that don't have a 2 letter code (eg: haw for Hawaiian). 

The Three Letter Windows Language Name is mostly used for in-box locales for things having to do with our build process, so you can pretty much pick anything.  Mostly I just use the ISO code, but note that Windows tries to keep this value unique.  Note:  Do NOT use this 3 letter windows code in your application, instead use the ISO standard codes.