A major question for translators to decide in this and the following verses is where the direct speech of the LORD finishes. Moffatt and Good News Translation finish the LORD’s words at the end of verse 10 (supported by Petersen). Bible en français courant, Die Bibel im heutigen Deutsch, and Parola Del Signore: La Bibbia in Lingua Corrente carry them to the middle of verse 11 (supported by Merrill). Revised Standard Version carries them down to the end of verse 12. New International Version, New Living Translation, Contemporary English Version, Beck, and Traduction œcuménique de la Bible carry them down to the end of verse 13 (supported by Amsler, R. L. Smith). Many other versions do not use quotation marks and so are not clear on this point. No available commentary discusses the question in any detail, but we may make the following observations:
(1) The Hebrew states clearly that verse 10 is the words of the LORD, so this is not in dispute.
(2) The occurrence of “says the LORD” at the end of verse 10 could possibly be interpreted as an indication that the LORD’s words finish there, but it does not have to be.
(3) In the Hebrew of verse 11, the LORD refers to himself in the first person, my people … I will dwell, which strongly suggests that he continues to be the speaker.
(4) The words at the end of verse 11, you shall know that the LORD of hosts has sent me to you, may be the words of the prophet, though they could equally well be treated as an aside that interrupts the LORD’s words (compare with verse 9).

When these points are taken into consideration, we can add the following comments:
(5) On the whole it seems best not to follow the Greek in verse 11, and end the LORD’s words at the end of verse 10. This is the choice of Moffatt and Good News Translation, and is probably the intended choice of Bible de Jérusalem, Jerusalem Bible/New Jerusalem Bible, New American Bible, New English Bible/ Revised English Bible, and New Jewish Publication Society’s Tanakh.
(6) If the traditional Hebrew text is followed, as Hebrew Old Testament Text Project and Critique Textuelle de l’Ancien Testament propose, the LORD’s words could be taken to extend as far as in the midst of you in verse 11. Versions taking this view include Bible en français courant, Die Bibel im heutigen Deutsch, and Parola Del Signore: La Bibbia in Lingua Corrente. The problem is that this view leads to an abrupt change of speaker in the middle of the verse.
(7) There is no evidence in the Hebrew text that the LORD’s words could not include verse 12. Therefore translators are recommended to follow the punctuation of Revised Standard Version.

Many nations shall join themselves to the LORD: This indicates that the LORD’s blessing will extend beyond the limits of the Jewish nation. In more natural English, Good News Translation expresses this as “many nations will come to the LORD.” Compare Isa 2.2-4; Micah 4.1-3. Translators may also say, “People from many countries will become the LORD’s people.”

In that day refers to a period of time longer than 24 hours. Good News Translation translates with the more general “At that time.” Compare the notes on Hag 2.23. Translators could also say “from that day [or, time] onwards.”

And shall be my people: The foreign nations will turn to the LORD and share the same kind of relationship with him as the Jews.

And I will dwell in the midst of you: In Hebrew these words are identical with those in verse 10, and you refers back to “daughter of Zion” there. This clause may indicate a position of special privilege for the LORD’s people when the gentile nations turn to him. If so, then it may be better to translate the opening conjunction as “but” rather than and. In this clause there is an abrupt change from the previous two clauses in the way the LORD is referred to. He now speaks of himself in the first person, as in verse 10. Some translators may prefer to keep this abrupt change in their translation. Others, however, may decide to follow the example of Good News Translation, and change my people and I will dwell to “his people” and “he will dwell.” Another possibility is to use the first person in the first half of verse 11, and say, “Many nations will come to me [rather than “to the LORD”] and will become my people.” But if this possibility is accepted, it may cause some confusion with the use of me in the last part of the verse to refer to the prophet.

You shall know that the LORD of hosts has sent me to you: These words are not identical with those of verse 9. In verse 9 “you” was masculine plural, whereas here it is feminine singular. Here also the words to you (again feminine singular) are added at the end. In this context you can refer back only to “daughter of Zion” in verse 10. But since “daughter of Zion” is a way of speaking collectively about the people of Jerusalem, it may be necessary to translate you as plural in many languages.

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 .