Hi Juraj and Brenden,
I followed Juraj's suggestion and added the appSetting in the web.config file of the Admin and the en-US version of the Resource strings was removed (or is not visible any more).
Now, we have the el-GR and the en-GB cultures for inserting Resource strings, which was what we needed.
On the other hand we still get the "The system does not contain a localization pack for the selected culture. You can download additional localization packs from http://devnet.kentico.com/download/localization-packs" message in the Resource strings area. Can we do something to remove this message because it will be frustrating for the authors?
@Brenden: We were talking about the web site cultures and resource strings. As it seems the default culture (in regards to Resource strings) is configured by the Admin application, even if you do not want to use it in the web site application.