I tried converting a wav file with a greek filename to flac, by using the FLAC Front End, It made an error : Run-time error '53': File not found.
When i give as input a wav file with latin caracters there is no problem at all.
So i'm quite sure the problem comes with wav files containing greek caracters in their name.
I'm using Windows 7 Home Premium, in english.
Here is some feedback i got from hydrogene audio :
flac.exe is being invoked through a command shell, which is like a DOS prompt, so the characters are converted to what's valid there. This will only be 8-bit characters, whatever your system's ANSI set is (Windows-1252 for most folks). So probably things like é and ä will work but Greek is not an option unless you're using a Greek version of Windows.