The stranger that sojourneth, to wit, a proselyte, not any stranger, as some understand it. For since it is confessed all the other ceremonial laws do not oblige them, and that where the name of stranger is put, as here it is, it generally speaks of a proselyte, it is more reasonable to take it so here, than without any reason or evidence to make this a particular exception from the general rule.
Related Commentaries of Numbers 19:10
Numbers 19:10
10 And he that gathereth the ashes of the heifer shall wash his clothes, and be unclean until the even: and it shall be unto the children of Israel, and unto the stranger that sojourneth among them, for a statute for ever.