Names originating from Spherus Magna have always had a Greco-Roman feel, like Vastus, Malum, and so forth. Some of them come straight from Latin, like "magna" for "great."
So, since Umbra was presumably named by the Great Beings, and since his name sounds Latin, not Polynesian (Matoran), does his name actually come from the Agori language, not the Matoran language?
Does his name indeed mean "shadow," as it does in Latin?
Yes, it is Agori.
No, it does not mean shadow in Agori. Since his powers are all light-based, shadow wouldn't really make sense for a name.
Quote from Bso1: ”Although he is a being of light, he was given the name "Umbra" to confuse enemies as "Umbra" means "the darkest part of a shadow.”
Could this mean Umbra means shadow in Agori? We know it means "the darkest part of a shadow” in Matoran language at least.
The origin of that factoid has a bit of a convoluted history.
Back when Umbra was first revealed, people were confused why he had a "dark" name, since he was a good guy with light powers. Besides being a Latin word, "umbra" is used in English as a scientific term for the fully-shaded part in the middle of a shadow. That's the definition you get when you look up "umbra" in the dictionary. However, the term umbra is taken from the Latin word umbra, which just means "shade" (it can also be used in the sense of "ghost," which is also very fitting for Umbra's character, since he can turn into pure light).
Anyway, back in 2006, everyone assumed that Umbra's name literally meant "the darkest part of a shadow" in Matoran (which actually doesn't make any sense, because although that is its meaning in English, there's no reason to believe that it would have the same meaning in Matoran). When someone pointed out that "umbra" has dark connotations, Greg justified it by saying that it was a trick to confuse potential enemies.
Original quote: https://greg.thegreatarchives.com/2003-2008/page126#post5012
In Latin it could mean:shade, ghost, shadow, phantom, darkness, dark, night, dusk, obscurity, shelter, cover, leaves, foliage, illusion, protection, security, care, safety, guard, preservation, silence, stillness, quietness, hair, quiver