Is not clean: this does not refer to being physically unclean but to ritual impurity, which would prevent the Israelite man from fulfilling his duties as a member of “the assembly of the LORD.” In languages where the idea of ritual impurity will be difficult to express, we may say something like “he is unfit to worship [or, have fellowship with] Yahweh.” Contemporary English Version has a similar rendering: “makes a man unclean and unfit for worship.”

By reason of what chances to him by night: this refers to “a wet dream,” that is, an emission of semen (New Revised Standard Version “a nocturnal emission”). This is dealt with also in Lev 15.16-17. Every language has its own way of referring to this; but translators should be careful not to use language that is offensive to the reader or hearer.

He shall go outside the camp, he shall not come within the camp: that is, “He must leave the camp and stay away from it the whole day” (as the next verse makes clear).

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 .