I think it is safe to say that the standard view today among Matthean scholars is that "all nations" in Matthew 28:19 includes both the nations (non-Israel) and Israel. It is often read as a revision of the earlier exclusive mission to only the "lost sheep of the house of Israel" (Mt 10:6). The evidence in favor of either view is even, although the linguistic evidence seems to nudge in the direction of an exclusive reading (see the 1975 CBQ article [37: 359-69] by Hare and Harrington "Make Disciples of all the Gentiles" [MT 28:19]). By the way, the article is very dated in terms of its theological outlook, and their argument is not as strong as they would have you believe, but the evidence is solid. It seems to me that the determining which way to read the "all nations" in Matt 28:19 has everything to do with one's view Israel in God's purposes.
For my own view on this question you can see my discussions in Matthew's Messianic Shepherd-King
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