You can do it this way:
how to add languages to your phone - Top Sony Forums

Or

by flashing, using XS++
XS++ 3.1 Guide-Flashing,Customizing,Recovering - Top Sony Forums

You firstly need to see the language pack you need:
Top Sony - Languages Description

Next go to download section click W580i
Downloads - W580 - Top Sony Forums
And then choose one, and download the MAIN and FS you will also need the custom pack.