The other comment is not quite right, a ‘language register’ is not a dialect, even though apparently the whole classification is difficult and imprecise due to the nature of languages as a continuum. A ‘language register’ means a variant, choice of words, that are used in specific situations or settings: https://en.wikipedia.org/wiki/Register_(sociolinguistics)
So, afaiu an ‘informal register’ would be something like brospeak or language spoken at home and among friends, contrasted to that spoken with strangers and at work. But I don't know what the situation is in German. With English and Russian, every generation and each subculture invents their own slang just to differentiate themselves―can't imagine how any country would avoid developing informal language, considering the existence of Oktoberfest.
So, afaiu an ‘informal register’ would be something like brospeak or language spoken at home and among friends, contrasted to that spoken with strangers and at work. But I don't know what the situation is in German. With English and Russian, every generation and each subculture invents their own slang just to differentiate themselves―can't imagine how any country would avoid developing informal language, considering the existence of Oktoberfest.