setting "utf-8" is fine in the language file. (don't forget to save it without the BOM, or as "ANSI as UTF-8").
If the wii can't print them correctly, then it's a font problem (it's not complete with full characters).
Japanese has/had to use a different font.ttf to see kana and kanjis.
Try the beta release (r1108), it now include an option to use the internal Wii font (instead of the included in USBloader).
You can enable/disable system font usage in the GUI settings of the loader.
If the internal Wii font doesn't include greek fonts, you'll have to use a different font.ttf, place the file into the same folder of boot.dol, or in the themes folder (I don't remember the correct one).