Default Languages missing in custom firmware

There are many setup languages missing in all custom firmwares, in my case Icelandic.
The official kitkat rom has Icelandic, but the custom roms look so much better !
I can set the keyboard in my language but not the phone, is there any way to add this afterwards ?
I would be extremely happy if it could be added if future rom builds ( and all custom rom using Icelanders )