The Levites shall surround the king: Since the previous verses indicate other duties for some Levites, Bible en français courant renders The Levites as “The other Levites.” Good News Translation makes explicit why the Levites surround their king by using the verb “stand guard.” Contemporary English Version is similar with “protect,” and so is New Living Translation with “form a bodyguard around.” The king refers to Joash, which Contemporary English Version makes explicit by saying “King Joash.”
Each with his weapons in his hand: The weapons were most likely “swords” (Good News Translation, Contemporary English Version). Swords are normally kept in their sheathes when there is no immediate danger. But the extreme threat to the safety of Joash in the situation foreseen here requires that the swords be drawn and ready for action. According to Contemporary English Version, the swords must be “ready to kill.”
And whoever enters the house shall be slain: The house refers to the Temple. The passive verb shall be slain may be easily transformed into an active one since it is quite clear who the agents are. It is the protective guard of Levites who are to kill any intruder. So this whole clause may be rendered “The guards [or, Levites] must kill anyone who enters the Temple” or simply “They must kill any person who comes into the Temple.”
Be with the king when he comes in, and when he goes out: The Levite guards are to stay with the king all the time. When he comes in, and when he goes out is a common Hebrew idiom meaning “wherever he goes” (Good News Translation, New International Version, Revised English Bible, New American Bible, Knox) and should probably not be translated literally in most languages.
The long quotation in verses 3b-7 may be made into indirect speech in those languages where direct discourse is not acceptable here. A possible model for this is as follows:
• Jehoiada told them that the son of King Ahaziah who had died must now be their ruler, just as the LORD had promised concerning the descendants of David. 4 Then he gave them the following rules: A third of the priests and Levites who go on duty on the Day of Rest must guard the Temple gates, 5 a third of them must guard the royal palace, and a third must guard the Foundation Gate; and all the other people must gather in the courtyards of the Temple. 6 No one should enter the Temple except the priests and the Levites on duty; they may enter because they are ceremonially pure, but all the others must observe the LORD’s command not to enter the Temple. 7 The Levites must stand guard around the king, each one with his weapons ready. They must stay close to the king wherever he goes. Anyone who enters the Temple must be put to death.
Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Chronicles, Volume 1. (UBS Helps for Translators). Miami: UBS, 2014. For this and other handbooks for translators see here .