In Revised Standard Version verses 49-51 are one sentence, which must be broken into several smaller units.

A nation … from afar, from the end of the earth: this is a normal way of speaking of a country a great distance from Israel.

As swift as the eagle flies: it does not appear that the swiftness is the main point of comparison, but rather the manner in which an eagle swoops down on its prey. Good News Translation has “will swoop down on you like an eagle,” and New Revised Standard Version, New International Version, Revised English Bible, and New Jewish Publication Society’s Tanakh have the same kind of statement. See a similar use of the figure in Jer 48.40; Hos 8.1. In cultures where eagles are unknown, we may use a similar bird if that is available, or say something like “just like a large bird of prey swoops down.”

A nation whose language you do not understand: this is natural, since these people would come from a far off land, and the Israelites would never have had any dealings with them. Contemporary English Version has a helpful model: “foreigners who speak a strange language.”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .