"menschlich" can have a tiny bit of a moral connotation, e.g. "Frankenstein's creature showed human emotions", but that's really just about the difference between humans and allegedly "unfeeling beasts of the wild", so I don't think it's anywhere close to the English meaning of "mensch".
More commonly, "unmenschlich" would translate as "inhumane" (e.g. conditions in a prison), but more common than that, I think, would be "menschen(un)würdig", literally: "(not) worthy of / fit for a human" ("We need humane housing for the refugees").
While "unmenschlich" can refer to persons as well as things ("he is an inhumane person"), "menschenunwürdig" can only refer to things ("inhumane conditions").
And then there's "Menschlichkeit", which means a behaviour or "moral state of mind" that's about e.g. helping those in need, showing understanding and sympathy ("he showed Menschlichkeit to the homeless man").
But "Mensch" in itself only ever means "human being" as opposed to animals (on a purely biological, not on a moral level) or Vulcans/elves/demons.
Du bist der Mensch would mean "you are the human" -- with the definite article.
In English, what used to be one word ān in Old English split into two words in Modern English: a numeral "one" and an article "an" (which later lost its "n" except before vowels).
German did not have this split, and so ein can translate to "one" or to "a(n)".
Nor did German split "the" and "that" -- it uses der, die, das for both meanings, unlike English. ("the" was from the old masculine form, "that" from the old neuter form of what was "the same word", but later those two forms got specialised as the gender system disappeared in English.)
Nor, in fact, did German split "to" and "too" -- which also used to be one word, but where some meanings are now spelled as "too" to distinguish them. Thus German zu covers some (but not all) meanings of "to" and some (but not all) meanings of "too".
On the other hand, German split off dass from das -- so those both can translate into "that" in English, and you (and German children!) have to know grammar in order to know when to use which spelling: much as English schoolchildren have to know grammar in order to know when to use the spelling "to" or "too", since they're (often) pronounced the same.