The whole Law here is, of course, the Jewish Law, not understood as a legalistic system but as an expression of God’s will. The verb translated summed up is literally “fulfilled” and can mean either “to summarize” (Good News Translation, Knox, Jerusalem Bible, New English Bible), “to complete,” or “to make perfect,” as some commentators suggest. If the meaning is “to summarize,” the content of the whole Law can be summed up in one statement. If the meaning is “to complete” or “to make perfect,” the intent of the Law can be carried out in the activity of love.
The whole Law may be rendered as “all the laws” or “all the laws given through Moses,” and an equivalent for summed up in one commandment may be “are equal to just one commandment” or “to one law.” Accordingly, the introductory statement in this verse may be translated as “for all the different laws together are really equal to only one law.”
The quotation is from Leviticus 19.18, and once more it is taken from the Septuagint. In its Old Testament setting, it is simply a command for Israelites to love their fellow Israelites; here, it is understood as a command for Christians to love one another, regardless of their race or nationality. Jesus also applies this quotation in a similar sense (Luke 10.25-37, where the one who does the loving is a Samaritan and the neighbor is a Jew).
As you love yourself is literally just “as yourself.” What the quotation is saying is that you must love your neighbor as you love your own self. Good News Translation makes this clear by supplying you love. In order to make this commandment applicable to all persons rather than simply a specific command to a particular individual to love a particular neighbor, it may be necessary in some languages to employ a plural form, for example, “you all must love your fellowmen as you love your own selves.”
Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on Paul’s Letter to the Galatians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .