Hero

This is the actual first feature film in Scots Gaelic. Other later films have tried to make the same claim, such as "As an Eilean" (1993) and "Seachd: the Inaccessible Pinnacle" (2007), even though both of those have entire scenes in English which might disqualify them, unlike "Hero". On the flip side, those films feature native speakers whereas "Hero"'s actors all had to learn the language.