invalid resource directory folder in android when generating folder for strings

[JennyDiver]

I created the folder /values-el_rGr under res in order to place the strings.xml for the greek language and I get the error:\[code\]invalid resource directory folder\[/code\]I read prevous answers but you do not suggest anything, only that resources names bust me only these:resources. But I have read in these 2 tutorials Link1 and Link2 that I can generate that folder and use it for localisation. So what I do wrong?