I don't have a solution, but I can offer some hope. Greek Cyrillic letters do appear properly in INT ArchiCAD 19 using Windows 7. Might be an issue with the educational version?
David Collins
Win10 64bit Intel i7 6700 3.40 Ghz, 32 Gb RAM, GeForce RTX 3070 AC 27.0 (4001 INT FULL)
Thank you very much! The guide worked, although It was a bit different on Windows 10.
I searched for something like this but strangely I didn't find it on Google.