Singers do not sing the language they speak

Something you probably won’t hear too much today, Paul Robeson, an African American singer, singing in Yiddish:

https://m.youtube.com/watch?v=hQ3lRWyVX9A

Mazel tov, Paul. Happy Chanukah.

Tell me what language Paul Robeson did not sing?
He also sang his own language.

Most opera singers are are proficient (if not fluent) in 3-4 languages; typically Italian, German, French and English.  Many European artists are fluent in more.  My younger daughter sings opera and took three weeks of diction lessons last year before performing  Janáček's The Cunning Little Vixen in Czech.

Most pop artists that sing in a non-native language simply learn (read) the words phonetically.

Magma - sing in their own language.
what is the story with Joe Dassin: he does seem to sing in his native language, no accent I can pick up with French being my second foreign language (English being third)...