The LORD will become king over all the earth: This sentence could be taken in the sense that the LORD is not king now but will become so in the end time. This is not the prophet’s intended meaning, which is rather that the LORD will become visibly king over all the earth in a way which is not yet the case. For similar expressions, see Psa 93.1; Psa 97.1; and 99.1. In cultures where there is no tradition of monarchy, and so little or no royal terminology, it may be necessary to render king as “great chief,” “honored leader,” or something similar. In this context the Hebrew word for earth (ʾerets) clearly means the whole world, not just the Holy Land as in the following verse. (The close link established in this verse between the end time and the LORD becoming king helps to explain why people were so interested when John the Baptist and Jesus began to preach that the kingdom of God was imminent; see Matt 3.2; Matt 4.17; Mark 1.15.)
On that day is not in this case a discourse unit marker, because it does not come at the beginning of the sentence. It is simply a time phrase emphasizing again that the events mentioned will occur in the end time.
The LORD will be one: The Hebrew words are the same as the last two words in Deut 6.4. They emphasize one of the key elements in the Jewish faith. As in the previous sentence, translators should avoid giving readers the impression that the LORD will at some point in the future become something which he is not already. The idea is rather that he will be universally recognized as the only true God.
And his name one: This statement sounds a little odd when translated literally into English. The meaning is that the LORD’s name (that is to say, his person) will be the sole object of worship throughout the world. Some Jewish scholars also take this verse to mean that the ban on pronouncing the sacred name YHWH will be lifted.
In many languages it will be helpful to restructure the second half of the verse to express the meaning more clearly. Possible translation models are “Everyone will acknowledge [or, worship] him as God, and will know him by the same name” (compare Good News Translation) and “He alone will be worshiped as God and his name alone will be acknowledged by everyone” (so Contemporary English Version, Bible en français courant). Other possibilities are “There will be no other LORD beside him, and nobody will worship any other God on the earth” (Die Bibel im heutigen Deutsch, 1. Edition), “He alone will be God on that day, and to him alone will people pray” (Die Bibel im heutigen Deutsch, 2. Edition), and “Everyone will honor and acknowledge him alone as God” (Parola Del Signore: La Bibbia in Lingua Corrente).
Quoted with permission from Clark, David J. & Hatton, Howard A. A Handbook on Zechariah. (UBS Helps for Translators). New York: UBS, 2002. For this and other handbooks for translators see here .