Meshed up language characters

I' ve installed-upgraded a PC into Windows 10 Creators Update 15063.138. The problem is that when I use an installed program and try to use the Greek keyboard on its options or input fields, the characters are something like "|||||||..." and when I switch to native OS language US, the characters are as expected. The problem doesn't exist on previous 14393.1066 edition as I have the same program installed on another PC and the characters present as expected on BOTH languages. I tried and switched the system font using a notepad editor and the registry, I installed Greek language pack and used it as main system language and run the program, but the problem remained! I can't seem to be able to use properly the Greek keyboard characters. I have to mention of course that when I open a notepad txt, or a word document, the Greek characters are there!! I used compatibility settings, older editions of Windows, but it drives me nuts! Can anyone help me ?? I have to point out that the program, whose menus are in US English, was installed and re-installed on both OS languages on Creators Edition, but I can'get it to work..

Flash update: Neither Microsoft's engineers nor their Remote Desktop support (via LogMeIn) couldn't resolve my issue..They performed the same steps as I did (I watched them..) they even created another user profile, in case mine was corrupted, but...NOTHING was repaired... Too bad..I'll have to wait (??!!) in case another update resolves my problem..They even proposed to revert to my previous Windows 10 edition (14393.1066)!!! Of course I knew to do that, but I didn't want that to happen.. I expected MORE from their support, as I performed every action they proposed,before they did...