There's no direct equivalent to "understatement" in spanish. Please do not use the "google translate" variants, as they're not correct. "Subestimación" is an eyesore word, derived from the verb "subestimar", which means to underestimate (NOT understate). "Atenuación" isn´t a valid translation either. The best way to express the spirit of "That´s an understatement" is with the expression ´quedarse corto´ (literally means to fall short), which you would have to adapt and conjugate appropriately, depending on the context. For example, if I said "To say that I´m tired would be an understatement", I´d say in spanish as "Decir que estoy cansado se queda corto", or "Si dijera que estoy cansado me quedaría corto".

Well, I found that it translates as :"Es un eufemismo" o "es una atenuación"

Eso está sobreentendido.