Please don't use greek text in your fanfic [PJO]
I have read a couple of fics recently that have whole discussions between characters in 'Ancient Greek' using actual greek characters and I basically just skipped it. This wasn't meant to be a discussion that the reader didn't understand, the writer did put in translations in hover text boxes, which was nice, but I still didn't like it.
I've also seen it used in single words, like names, but I'll skip it then too. If I can't read the word in my head, I'm not reading it. I am not familiar with the letters so I wont recognise if it's used again, you could be using a bunch of similar looking words, I'm not paying enough attention to notice.
If you want characters to speak in a different language and be understood then just use italics. The only time I would expect to see greek writing would be if it's not meant to be understood and even then I prefer english letters.
Greek letters are too disruptive to my reading experience. I'm not going to waste the limited time I have for reading fanfic, trying to puzzle out what you wrote. I want to immerse myself in the story, not click a bit of text, read that, click away, read to the next line, then click a new bit of text, and so on.
There are other ways a writer can indicate that someone is speaking another language.