Yoel 3:1 gives an outstanding prophecy mainly talking about Israel. When it talks about pouring the Ruach Hakodesh on all flesh, who is it talking about?

There’s a difference of opinion. Some commentaries say it’s referring to all of Israel, the classic commentator Ibn Ezra adds: who are in the Land of Israel, for as a rule, there’s only prophecy among the Nation of Israel in this Land. On the other hand, the Metzudot David commentary explains that the very inclusive term: “on all flesh”, may include all of the remaining people (including gentiles) who “pass” the Judgment Day (which Yoel details in the continuation of that prophecy), successfully.