regarding what the imam says before starting prayer (to make the line straight etc)

Recently I have been wondering regarding something that I've been hearing since childhood. when we stand in the congregation for the obligatory prayer making the lines, the imam says in arabic a few phrases. i can make out that what he says is to make the lines in accordance with the sunnah (*our mother tongue is not arabic*) my question is that is it a must to say that in arabic everytime before we start the prayer?, or is it sunnah?, or are they doing it just to remind us to make the lines straight as its kind of like the responsibility of the imam.