Yeah! Tell me about it...
Have tried a few things:
1. AdminMessages_el.properties with ISO-8859-7. Did not work either.
2. I moved the AdminMessages_el.properties (with \uNNN encoding) to AdminMessages.properties and everything started working.
3. I moved the AdminMessages_el.properties with ISO to AdminMessages.properties and it displayed garbage.
For some mysterious reason, although the necessary XML files are all updated and correct, the Admin-app does not switch locale automatically.
I will have to stopkonakart, move the file and startkonakart again. I can certainly live with that, since nobody needs to change the language
of admin very often. Still i remember the times (around version 2.2.x.x and 4) when i could switch the locale from "Languages" and only needed to log-out.