Imrazôr is a descendant of a family of
the faithful who settled in Belfalas before the downfall of
Númenor and were said to be related to
Elendil. (One assumes, that they came to
Middle-Earth after the removal of the Faithful, by
Ar-Pharazôn kathuphazgânun from Andúnië to Romenna.)
I do not know why he was called the '
Númenórean' it could be that he was very much like the Númenórean lords of old (Of course the descendants of Anárion would have been like the Númenórean
peoples of old, and of pure blood but it would have been seen as unusual for a lord of a Gondorian fief. Of course a lot of the "best" Númenórean blood was lost in the Last Alliance and Battle of Erui.
I too think that it is odd that he would have a
Adûnaic name, esp. in consideration of the statements in
Of Dwarves and Men (HoME 12) and
River and Beacon Hills of Gondor that Adûnaic was abandoned by the remnants of the
Númenóreans who held the coasts because of the fall of
Númenór and the Faithful used
Sindarin as their first tongue, but eventually, they had to use
the Common Speech to converse with other Men and Sindarin became the language of the Númenórean nobles. (The Common Speech spread widely in the T.A and became known as
Westron or Annunaid amongst the
Elves.)
It may be that one of Imrazôr's parents or grandparents originated from
Umbar, and he was given an Adûnaic name by the relation who hailed from Umbar , and he was chiefly remembered by that name. According to the
Heirs of Elendil (HoME 12) Imrazôr was born in T.A 1950. This was 140 or so years since King Telumehatar Umbardacil re-took Umbar, or maybe Adûnaic briefly prospered in Pelargir (Though of course the
events of the civil war took place about half a millennium earlier, it could be that the removal of Adûnaic was slow, there was also little generation gap between the two events) or the adjoining coastlands since Castamir was popular amongst the people of the coasts (He was Captain of the
Ships at Pelargir and he had removed from
Osgiliath to Pelargir and it could be that Adûnaic was spoken by him and his people, since he had a lot of support from Umbar and resided in Southern Gondor in a coalition of rebels who may have took up Adûnaic in rebellion), where it is probable that Adûnaic names remained there since it was in Gondor's influence for a short time and it is credible that the conservative-minded
Black Númenóreans kept Adûnaic names. Though, rather interestingly, most of the well-known Black Númenóreans that we know of have
Quenya names (Fuinir, Herumor, Sangahyando Angamaitë) though of course these may have been the names by which they were known in Gondorian lore and they had Adûnaic names that the Gondorian scribes who compiled the Appendix didn't know of.
Imrazôr was obviously acquainted with Sindarin, since then how could he have communicated with Mithrellas. (Who must have spoken Sindarin even though Nimrodel spoke the archaic Sylvan tongue) and he also gave his children (Galador and Gimlith
Sindarin names) and it is probable that Sindarin was the main language of his household, as he was a Gondorian noble.