Hi everybody,
to be honest in the end I have chosen to flash my mobile. I didn't think that it will be soooo easy. I really appretiate what I found on this forum. I have another question ;) : Is there any way to make backup of whole firmware in the phone? ... just in case when you need to put it to service...

to the greek: if you have read the link above than you should be able to add your native language. It's just a little more of editing of the *.xml files ;) I;m sure that you'll be able to do it.