Welcome ,I will give a full and clear answer to your question, please make sure to reply to any questions I have. They may be essential to my reply.
Hi , on the login in screen is the Flag shown? , you can switch keyboard layout if so by clicking on the flag and type your password .
If you can login , try booting to safe mode and make the changes as need be in safe mode ,
To boot safe mode , Hold shift key while powering on and keep holding .
You may need to turn on Show input menu for login screen in the accounts control panel , under Login Options .
Or change your User login password in accounts , to use a password the at is setup while in Greek the keyboard layout etc .
Does this help .