We start the Haggada, and with it the core of the Seder night, with the mitzva of Maggid (the telling of the story of the Exodus), which begins with this invitation to join the Seder. The language of this paragraph is Aramaic, because this was the vernacular (spoken language) at the time the Haggada was written. If this invitation is to be genuine, it is important that it is stated in a language that is understood.