Third Nacelle wrote:
Neither. They'd be a citizen of the United Federation of Planets, as they were all along. They never emigrated. They just moved.
The phrase "Federation citizen" has definitely been heard on screen, I just can't remember when.
While not completely sure, I believe that came from Picard, but Picard is likely one of the rare people on the show (there are a few others) who would even use a term like "citizen of the Federation."
Captain Kirk might say the "I/We are from the Federation" in a official capacity, but it's perfectly clear that he thought of himself as a Earthman. Deanna consistently referred to herself as a Betazed, even with her mixed Betazed/Human ancestry, I can't remember Deanna ever personally calling herself a "Federatista" or some such.
The people of the United States of America refer to ourselves as "Americans." There is no
sign on the show that there exists any common identifier
for people from worlds that are in membership with each other in the UFP.
Federation citizen would be a informal colloquialism, that has no real meaning. Somewhat like the current "citizen of the world," "citizen of the Federation" would be a charming expression that has no official meaning.