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.

Intercosmos. URSS - India.

XX Anniversary Intercosmos. Cuba

Intercosmos Hungary

URSS Beautiful registered cover to Argentina

ИнтерКосмос was a Soviet progam that included manned missions to space. The participants countries were Soviet allieds from the Warsow Pact and countries like France and India. Between 1978 and 1988 there were fourteen internacional crews.

On May 26, 1980 Bertalan Farkas from Hungary went to space in the Soyuz 36 mission. These stamps were dedicated to this mission.

Intercosmos - Mongolia

ИнтерКосмос was a Soviet program that included manned missions to space. The participant countries were Soviet allieds from the Warsow Pact and countries like France and India. Between 1978 and 1988 there were fourteen international crews.

On March 23, 1981 Zhugderdemidiyn Gurragcha from Mongolia went to space whith the Soviet cosmonaut Vladimir Dzhanibekov in the Mission Soyus 39.

Beautiful register cover from URSS to Argentina (1990)

Three stamps were dedicated to the "Intercosmos URSS-Mongolia" with face values of 6, 15 and 32 kops., issued in 1981.