This problem is caused by the files' metadata being stored using incorrect encoding (non-Unicode). The language setting in iTunes or fonts have nothing to do with this.
Most likely the problem is because the files have been encoded using an old windows program. I saw similar issue after adding a couple of mp3's that were ripped with windows media player.
To fix the problem with the specific files without re-entering all information manually, you have to find a 3rd party software that allows to convert ID3 tag encodings, for example
http://unicoderewriter.sourceforge.net/. You should also research what encoding was used originally so that you set up conversion correctly. For example, if the files were ripped on a Greek PC, the encoding would be windows-1253.