a language, and so the people who speak it: one of the language groups an empire counts among its subjects
This is the Aramaic counterpart of the ordinary Hebrew word for tongue (לָשׁוֹן), but in the Aramaic parts of the Bible it never means the organ in the mouth. All seven times it appears, it sits inside one fixed phrase of royal address: "peoples, nations, and languages."
Inside that phrase a "language" is a crowd of human beings, not a system of words. What the sentences say about them settles it.
- They fall down and worship a golden statue in Daniel 3:7.
- They tremble and are afraid before the king in Daniel 5:19.
- They live scattered across the whole earth in Daniel 4:1 and Daniel 6:25.
- They will serve the one given everlasting rule in Daniel 7:14.
A language cannot do any of that. People can. The word counts human beings by the speech they share.
The phrase itself is the empire's way of saying "absolutely everyone." Daniel gives it to Nebuchadnezzar's herald, to Nebuchadnezzar's own letter, to the account of Belshazzar's father, and to a decree of Darius. Then, at the turning point of the book, the same total sweep of humanity is handed to the one like a son of man. The crowd that bowed to a statue of gold is the crowd given to him.
It never travels alone. Its two companions are the plain word for a people (עַם) and the word for a nation (אֻמָּה). The three are not a careful classification of humanity into tidy categories. Piled up together, they simply mean that nobody is left out.