Umseting - Týkst-Grikskt - Schöne Ferien

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εις το επανιδείν is a bit formal but is the only accurate translation of Auf Wiedersehen

There's also 'αντίο' which means 'goodbye' and 'τα λέμε' which verbarim means (we'll talk later) and is a really informal equivalent of Auf Wiedersehen.