I did a search and found a few topics that this may duplicate, however they are of older snapshots. I apologize if this is a duplication.

I am using the new 1.6 pre-release. Literally just downloaded it yesterday.

I wanted to make a Resource Pack so i downloaded a default pack here:

http://www.minecraftforum.net/topic/1849144-13w24b16-resource-pack-template-no-adfly/

I then proceeded to create a custom language file and changed a few of the fields. I am sure the pack is not corrupted in any way. (I even copy and pasted the en_US pack and renamed it from scratch just to make sure i had not messed anything up.)

For whatever reason, my custom language pack is NOT showing up in the game, and yes the resource pack is loaded because my redstone dust is updated correctly. However if i go in and edit the fields contained in en_US and load it, my language changes take effect.

I cannot get a custom language file to load into the game though, it wont even appear in the scroll interface. I also tried placing it in the .minecraft/assets/lang directory but alas, it still does not show up.

Here is my .lang file and the line i changed was the top ones, and then:
menu.singleplayer=Singleplayer Test

It appears that Resource Packs are ignoring the languages AND the language packs inside .minecraft/assets/lang aswell.