Is Fez's country ever revealed?

It remains unknown. From Fez's Wikia page:

His secret country of origin is one of the longest running gags on the show. The reason that Fez's name and country of origin are never mentioned throughout the course of the series is that the creators could never decide on his origins, and eventually decided to leave it a secret. In an interview, Wilmer said he created an accent so no one would know what country he is from. Many theories about Fez's homeland have been produced by fans of the show, but since there is no answer that the creators can provide, it will not be known if these are true.

His Wikia details many clues dropped throughout the show, some of which are potentially contradictory. Some fans claim that he's from the Falkland Islands or Brunei based on some of these clues (see this long discussion), but with no formal approval, it remains a theory. It's best to just wait for the Word of God on the issue because, like the Simpsons' state, the mystery mainly exists just to mess with us. :)