well you can pratically do it, you copy the language that you want (hindi for example) to tpa\preset\system\language and modify the customize.xml by adding a new line under the allowed languages, like this
<allowed-language>the language that you want</allowed-language>
If you find some files named: "lng.dat" and "lng.lst" delete them, they will be regenerated... for using a T9 I think it may not work if you use a "language.t9" with bugs...