Sigmund Jähn. Thank you Lily !!

From Wikipedia:

Sigmund Werner Paul Jähn (born 13 February 1937) is a German pilot who became the first German to fly in space as part of the Soviet Union's Intercosmos program

In 1976, Jähn was selected with his backup Eberhard Köllner for the Intercosmos program. He trained in Star City near Moscow for the next two years, and flew on board Soyuz 31 (launched 26 August 1978) to the Soviet space station Salyut 6, and returned on Soyuz 29, landing on 3 September 1978. He spent 7 days, 20 hours, and 49 minutes in space.

During and after the flight, he and the socialist authorities of the GDR pronounced him "the first German cosmonaut", which was remarkable, as in those days the East German state normally stressed that their people were "GDR citizens", to distinguish themselves from West Germany.