Following are a number of back-translations as well as a sample translation for translators of Deuteronomy 33:21:

• Kupsabiny: “(He/they) chose for themselves/himself the land/area that was best of all,
  where it was prepared to belong to a leader.
  (He/they) honored the decisions/judgments of God
  among/concerning the people of Israel.” (Source: Kupsabiny Back Translation)
• Newari: “They chose the best place of all for themselves,
  the leader’s share has been kept for them.
  When they, the elders of the people, had their meetings,
  How the LORD’s righteous will had to be obeyed-” (Source: Newari Back Translation)
• Hiligaynon: “They chose the best land which is-fitting for the leaders.
  When the ones-who-rule of the people of Israel gather, they follow the something-that-needs-to-be-followed by the LORD and his regulations/statutes for Israel.’” (Source: Hiligaynon Back Translation)
• English: “They chose the best part of the land for themselves;
  a large share of the land, a share that should be given to a leader was allotted to them.
  When the leaders of the tribes of Israel gathered together,
  they decided that the tribe of Gad should have a large share of the land.
  The tribe of Gad obeyed the commands of Yahweh and the things that he decided that they should do.” (Source: Translation for Translators)