Thanks for the prompt reply. I used http://www.unicode.org/repos/cldr-aux/json/22.1/main/el_GR.json as a source. It contains the same value for both primary and alternative (short) translations. So, now I know that this is a CLDR issue. Thanks a lot for clarifying, hey, this mystery had been causing me quite a headache since yesterday...

And, just for the record, CLDR is right in this case, not MediaWiki. What's more interesting, CLDR has been submitting this value at least since 2009! Although I'm very keen in helping CLDR with translations, why should I with examples like this one? This needs to be resolved, so that people do not feel like they work in vain.

@Nemo_bis : elwiki gives "Αζερμπαϊτζανικά" while CLDR gives "Αζερικά" for "az". There are many examples like this. If MediaWiki was sourcing the language names from CLDR, wouldn't these values be the same?