Hello,
1) You need to translate both: XStore theme and XStore core plugin. We would suggest you to use Loco translate (Loco translate > Plugins > XStore core), it will be easier for you to edit files using backend translator than upload files everytime you change anything.
2) Translation file name was incorrect. It should be xstore-el.po, xstore-el.mo instead of xstore-el_GR
Regards