I've been tring to make sense of the manual (
https://images-eu.ssl-images-amazon.com/images/I/A1eT6dj73CS.pdf ).
As I read it, pressing the 'Menu' button (or possibly the 'Quick' button) should take you to a list of sub-menus, as follows:
Picture
Sound
Settings
Installation
If everything is in Greek, that might look like this:
http://www.upl.co/uploads/Greek1581528570.jpg
If it's in Bulgarian, it might look like this:
http://www.upl.co/uploads/Bulgarian1581528609.jpg
There appear to be two ways forward from there:
Method 1:
Select the 'Settings' option.
Then select 'Languages'. (It's likely to be the second item on the list. In Greek you might see Γλώσσες . In Bulgarian you might see Езици)
That should enable you to select 'English'. (If it's labelled in Greek it's likely to be Αγγλικά. If it appears in Bulgarian it's probably Английски)
Method 2:
Select the 'Installation' option.
Then select 'First time installation'. (That's likely to be the last item in the list. If it's in Greek it might appear as Πρώτη εγκατάσταση . If it's in Bulgarian you might see Първа инсталация )
That returns your TV back to its 'out of the box' state. (You'd then need to re-scan for channels, just as you did when the set was new, but hopefully everything would be in English again. Note that, after selecting that option, you might need to turn the set off, and then on again, for the changes to take effect).
Aren't grandchildren wonderful?
;-)