You need a multi CSC firmware to be able to change it.
CSC is the consumer software customisation and is specific to geographical region and carriers. It contains the software packages specific to that region, carrier branding and also APN settings for data connection, MMS etc for your service provider.
CSC has nothing to do with what languages the firmware supports and it will not add keyboard language support either.
Swyped from a distant star, the other side of the Galaxy on my Slim ICS powered GT-I9100©