Perhaps this is a ridiculous question, but since {-ngan} isn't listed in the official klingon suffixes, then how are we able to write {'elaDya'ngan} unless we have the specific word {'elaDya'ngan} in Ca'Non?
Nobody has pointed this out, but the out-of-universe origin of {ngan} is almost certainly that it is a good fit for the English ending "-ian" for Trek alien names (Organian, Denebian, Cardassian, Ligonian, etc.) and also happens to be the ending of {tlhIngan}. (One can theorise that {tlhIngan} was originally a demonym for people from a place named {tlhIng}, a name which survives in {tlhIng yoS}.)
Thus, the "people of" meaning probably came first, and the "inhabitant" definition for the standalone {ngan} probably arose out of the necessity of giving it *some* definition. I have no problem accepting {'elaDya'ngan} to mean "Greek", in the same way that {DenIbngan} means "Denebian". For the "inhabitant" meaning, I'd be more specific about the place: {tlhIng yoS ngan} "an inhabitant of Klingon district", {tera' yuQ ngan} "an inhabitant of planet Earth", {'elaDya' Sep ngan} "an inhabitant of the region/country Greece".
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De'vID