On Avicsync the Internal TTS Voices (tts_nua_ve1) do not work, or use the VocalizerEx or the Voice Greek Universal Unisex which allows you to use the Google voice which should be default on your device.

On Israel in addition to the options valid for Avicsync you can also use the Internal TTS tts_nua_ve1, if you do not find them there are several Greek users who will be able to post them to you.