Accents and strange patterns of speech are used in Allo Allo to denote when characters are supposed to be speaking other languages. This is mainly used for humor, of course, as the "accents" are stereotypical patterns of speech. I think that when Helga screams COME IN HERE! or the Germans shout or sound loud and nasty, they are meant to be speaking German. German characters are the only ones shown doing this although on a couple of occasions Rene and Madame Edith shout at German soldiers while in disguise. Living in a border area, it would not be unlikely Rene and Madame Edith would know a little German.