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)