Dear Fatemeh and Achraf!
Yes, this motto is great in English. But we have to think about the translations and this motto sounds strange in the latin languages and Hungarian. About 30% of the asgardians or more speak these languages. You cannot say that these translations are not important.
My native language is Hungarian, the Hungarian translation is the following:
"Egy emberiség, egy egység."
Egy = One
Emberiség = Humanity
Egység (it made from the "one" word like the latin languages) = Unity / unit of stg / credit on your phone / battery level
In this sentence the "one" word is three times and it is very strange. Unfortunately it sounds stuttering. The latin languages and the german have the same problem with this motto:
Italian: Una umanità, una unità.
Spanish: Una humanidad, una unidad.
French: Une humanité, une unité.
German: Eine Menschheit, eine Einheit.
I don't say that this motto is bad, but we need a little modification.