You should of posted this thread in the Language change section:
Upgrade - Debrand - Language change - Top Sony Forums

Anyways back to your phone, it's quite simple
You can use this way:
how to add languages to your phone - Top Sony Forums

Or you can do it the propper way which is:
Firstly look and choose which pack you want:
Top Sony - Languages Description

Then download, go to Topsony > download > W810i:
Downloads - W810 - Top Sony Forums
Download the MAIN and FS and you will also need the custom pack.

And then use XS++ to flash the phone:XS++ 3.1 Guide-Flashing,Customizing,Recovering - Top Sony Forums

I used this method to flash my W710i from Europe 2 to Hong Kong - Traditional Chinese.