Radak: The nations that will remain after the war of Gog and Magog, I will make their tongues clear. They will not mention other gods; all will call in the name of Hashem - "ba'Yom ha'Hu Yihyeh Hashem Echad u'Shemo Echad" (Zecharyah 14:9)

Radak citing Ibn Ezra: They will be reversed to serve Hashem alone in a clear language - Leshon ha'Kodesh. Hashem's honored name is called only in Leshon ha'Kodesh.

Malbim: Via the war of Gog and Magog, all will recognize Hashem's power and wonders. They will abandon their idols and accept on themselves Ol Malchus Shamayim; Until now they called to Hashem in Safah (a language), but it was Berurah (clear). It was mixed with vain Emunos and follies in matters of Elokus. Then, they will call in Safah Berurah - in His unity.

Radak: Until now they did not serve Him together. They had different groups, and different Avodos. Then, all will serve Him like the way that the Torah commanded.