Is there a manual in Greek language?
Manual for what?
If you mean a manual for the Garmin Connect Mobile Android app (that's the name of the forum, you are posting to), then I am afraid not, since there is no user manual even in English, as far as I know.
If you mean a manual for a certain Garmin device, then have a look at http://garmin.gr or contact your local support for the assistence, if you cannot locate it there.