Would it be possible to become a successful singer if you sing in a language that's not your main one?

Yes look at Abba

Yes many have.Add Shakira to the already listed artists.

The Scorpions are German and English is not thier primary language.. though almost all songs are in English ... and they were very successful in the 70s -90s

certainly, there are many examples of this like Celine Dion, Bjork, shakira. lisa Gerard of dead can dance mostly sang in a language that doesn't even exist.

Pretty common for opera singers.

lots of singers have done that

If you sing well or whatever

yes