I just figured out how to make a custom language on a Mac and windows. Here are the simple steps:
***Attention*** Make sure you backup all your Minecraft files before beginning! It is not my fault if you somehow manage to screw up!
Step 1:At the top of your screen you will see a menu with some options. Click the "Go" option. Then go down to where it says "Go to Folder..."
you want to put in: ~/Library/Application Support/minecraft (make sure thats exactly what it says). For windows you want to type in : %appdata% and go to roaming and open the .minecraft folder.
Step 2:Go to the Bin folder and go to where it says minecraft.jar. If you are on Windows then you will need some sort of extracting program (I recommend winrar). For macs this is already built in. Next you need to rename it minecraft.zip. Make sure it says zip! Then duplicate it. After you have a copy, rename your original minecraft.jar again. Open the copy.
Step 3: Firstly,you need to delete the META-INF folder! <---THIS STEP IS VERY IMPORTANT IF YOU WANT MINECRAFT TO WORK! Then go to where you find the "lang" folder. Open it and you will see many items. The One you want to edit is "en_US.lang". Again make a copy of it. Rename the copy "en_" and then put what 2 letters you want. I will be naming it: en_MY.lang (again just only change the last 2 letters and make sure they are capital). Now go down to the languages.txt and open it. Put in a new line of text and put this in: en_MY=*Insert Name Here* (*Insert region here*). Then close that out.
Step 4: Open the lang file you made using textedit or notepad. You should see many different things. Firstly, change the language name to the name you want, the language region to what you want, and change the code to the code you are using EX:en_MY. Now you can edit whatever you want! You can change the diamond sword to exclaimer or change the single player option to Forever Alone. You can do whatever you want!
Step 5: Once you are done editing make sure you save. IF YOU ARE ON WINDOWS YOU CAN JUST SKIP THIS NEXT PART AND JUST RENAME YOUR MINECRAFT.ZIP TO MINECRAFT.JAR. Then go back to the minecraft.jar folder thing and select all. Then compress it. After that, there should be a new item called: archive.zip. What you want to do now is rename this: minecraft.jar. Then just plop that in to your "bin" folder and replace the old one!
You should now have created a new language! :biggrin.gif: If you mess up along the way and want to start all over just force update your minecraft and it should be back the way it was.
If you have any questions just leave a comment and I will try to help out. HOPE THIS HELPED! :biggrin.gif: