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Did the creature really change into other identities or was it just able to convince it's victims that it looked like whomever they wanted at that given time? That way it wouldn't need to know Swahili or any other language as the victim's own mind would give it that ability!
JB
This^ The appearance that the creature assumed might not necessarily be someone in the prospective victim's memory, but just having features that coalesced into a form that the creature sensed they would find appealing. As you say, there would be no need for it to have knowledge of the person's native language, just putting phrases together that it knew the victim would also find compelling.
That's been postulated about the salt vampires before, and is the main reason I question whether they're the ones who had the advanced civilization on M-113. If any intelligence they're able to manifest is directly linked to whoever or whatever they're creating an illusion for, it's a false intelligence; a masquerade to lower the defenses of its prey, and little more. Crater seemed to take advantage of the masquerade, until the creature sensed he was no longer going to be able to help it. Then it tried to get McCoy to take advantage of the masquerade, until he saw its true form. Apparently it couldn't keep the illusion in place while it was feeding.
I don't think that this is really substantiated. We simply aren't shown any behavior or thought on its part that doesn't relate to the need to feed, the means that it uses get its fix, and that it seems to have no bounds in limiting the pursuit of its goal. It may well be conscience, aware, and concerned about other things, but given the cornucopia of full sized sustenance now available, any other matters may simply pale in comparison. I find it interesting that of the creatures featured on the series that can speak for themselves, it may be one of the few that doesn't really say anything about its nature, identity, ethos, or anything really other than the received thoughts that it uses for this single function. Even many of the sophisticated computers we saw, onboard or on an alien planet, often explained themselves, to some degree. Why not beg for its life by revealing its nature of the last of its species, that it will cease its murderous behavior (whether truthful or not) and will allow itself to be studied if an accommodation on the salt stores provided to it, satisfactory to both parties, can be reached? In a way, the character does somewhat strike me as Kirk describes it, roughly an empty cipher for Crater.
As to it being unmasked, finally, before McCoy, you're suggestion may be possible. My thought on the situation, was that concealing its identity no longer served a purpose, at least for anyone in the quarters. Spock knew what it was, Kirk was about to die, and as that intention was clear, its appearance being hidden from McCoy had outlived its usefulness, as both he and Spock were gong to have die as well, so the pretense (and possible the energy required to maintain it) was now superfluous.