yeah!! it is possible to add a language in stock rom
for this you need a rooted android.
if u have a rooted android then go to root explorer(say FX explorer) and go to system and then go to FONTS folder and copy here .ttf format file(language u want to instaal) download your language file by searching it on google.
after copying the file reboot yout phone and enjoy your language.
NOTE-before copying, don't forget to unlock directory by clicking on top right corner lock button(avaliable in FX ROOT EXPLORER)
Sorry for bad english.