Dave, Yiddish is a Germanic language, but it is not modern German. Its vocabulary is based on Early and Middle High German.
My Yiddish dictionary (Siegmund A. Wolf, Jiddisches Wörterbuch, Hamburg: Buske 1991) translates German 'bei' and 'schön' as Yiddish 'bay' and either 'schen' or 'schein'. Besides, in German the song title would be 'Für mich bist Du schön' or words to that effect.
Reinhard