I did this with Nokia E71-1 With a Product code of “BENELUX NL” (Belgium, Netherlands, Luxembourg : Dutch), and I’m pretty sure this will work with all other S60v3 and S60v5 Nokias.
It’s quite simple and all you’ll need is the phone’s USB cable and a little application called NSS (Nemesis Service Suite) i’m not sure if it’s legal to post a download link here so ask Mr. Google for that, Nokia Software Updater (you can get that from Nokia.com And a new Product code for the new Language (google that, eg : “N95 product code”).
Note : you’ll lose your warranty by changing your product code so don’t do that unless you really need to or your warranty period already ended in this case you don’t have anything to lose!
Now Connect your phone and wait for Windows to detecting it.
Open Nemesis Service Suite and click “Scan for new device” on the right-upper part of the window >> Click on the icon “Phone info” >> Click on “Scan” >> Change the product code to and mark the case “Enable” >> Press “Write”, and your phone’s product code will be changed.
Now Close Nemesis Service Suite and run the Nokia Software Update and update your phone, viola your phone will restart with the new language.
You can use the same method to De-brand your phone.