An American space traveler is called in English an astronaut. A French space traveler is called in English an astronaut (not l'astronaute). A Japanese space traveler is called in English an astronaut (not uchū hikō-shi). A Chinese space traveler is usually also described in English as an astronaut. So, why are Russian space travelers called in English cosmonauts?
I think because it derives from the Russian term kosmonavt,
- 1959, Englishing of Russian kosmonavt, ultimately from Greek kosmos (see cosmos) + nautes "sailor" (see naval). (Etymonline)
The distinction is that they are titles awarded by different space agencies. They both mean essentially the same thing and they both come from Greek.
- Astronaut - astro (star) + naut (sailor) - Cosmonaut - cosmos (space) + naut (sailor)
Cosmonaut is the Anglicization of the Russian word космонавт. As the "space race" was a point of national pride, neither country was about to adopt the other's terminology.
- Cosmonaut is used by the Russian Space Agency. Astronaut is used by NASA, ESA, CSA, and JAXA. The Chinese don't use Greek terminology. The use their own language and call the person "space navigating personnel".