Pi HaHiroth is the name of a place, and therefore it's proper literal translation is "Pi Hahiroth," just as the translation of Migdol (later in the verse) would be Migdol.
However, the ancient Biblical translation Onkelos translates it as "Pum Chirasa," which is translating the Hebrew word for mouth (Pi) into the Aramaic word for mouth. So it would then be translated "Mouth of Hiroth." The commentary of Rashi states that Pi Hahiroth was two tall, erect rocks with a valley between them, and the rocks are like the lips of the mouth opening into the valley.