Config Language problem

Hey, Good Morning.

I have a bug - question and i wonder if someone has the same problem.
I have an opencart 1.5.6.4 with journal theme and i use two languages , English and Greek. I set as a default the Greek language and i've checked (i ve created a log file and write the default language of site every hour) that the default language changes without doing something, i ve never changed the language and the default language changed from 'gr' to 'en' and i know that i didnt do anything cauze sometimes it changes after midnight like 3:00 am etc. I don't use any SEO module and i dont run any cron exept of my log file about default language. Any suggestion? Have you noticed something like that? It's really weird. Is it possible to be a server session problem?

Thanks in advance for any idea.

Is the value for the 'config_language' key in the settings table in the database being changed?