If somebody had amnesia and forgot everything and everybody, would they forget the language they spoke too?

Not usually. There are five kinds of anmesia. Most people with amnesia have a localized amnesia where they forget events from a certain time period. The kind you are refering to is called generalized anmesia. It is very rare, but they will forget their entire lives, however they still retain their habits, tastes, and skills. They would still even know how to read, just not remember getting their education.