Earlier quoted context omitted.
I'm a spaniard and mongo is an insult. [removed unnecessary snarky comment]
Okay, there are separate issues going on here; let me try to clarify: Is "mongo" the equivalent of English "retard", in terms of being a low-class insult that invokes a visceral reaction among the majority of the population? I didn't believe that at first; if so, why didn't anyone ever put it in Wikipedia? English has "retard" (in the pejorative sense): https://en.wikipedia.org/wiki/Retard#Other_uses And why doesn't…
Maybe you have to be culturally immersed to know those things. Mongo, mongol and mongólico are the terms you should research.