All credit for this tutorial goes to @tranced.
1- Open A2Uploader and go to tpa>preset>system>language
2- Right click on the folder language
3- Go to the folder where the A2Uploader is located. Once you are there go to the "ph_out" folder and look for the "language" folder. Open the "allowed_language.txt" file and edit the file adding just two letters according the language you're going to add. In my case I added the spanish: es. Save the file.
4- Now add(or copy) the es.lng(language) and es.t9(dictionary) to the language folder and then reupload again to the language folder in the A2uploader.
5- Do the same with the customize.xml file located in tpa>preset>custom. Read it out and then look for it in the "ph_out" folder. Edit it and add the a new line with the new language. Then reupload it.
6- Done!