Because your phone had CID53 you can't unlock for free-only some payed service like Ultimate Unlocker-see here Top Sony - Buy Ultimate Unlocker LOGs or here Unlock Phones by IMEI - Top Sony Forums and for language change you need to unbrand phone with this tutorial
Flashing with jdflasher - Top Sony Forums and to have Greek you need this 3 files:


MAIN - K770_R8BC004_MAIN_GENERIC_VI_RED53
FS - K770_R8BC004_FS_MEDITERR_RED53 and
CUSTPACK - HB1-06_Gen_CDA102568_4_R8A_GENERIC_MEDITERR


You will find that files in download section - firmware - K770 and custom files - K770 and for download see here How to get access to Download Area? - Top Sony Forums