Following are a number of back-translations as well as a sample translation for translators of Isaiah 28:21:

• Kupsabiny: “God shall punish you (plur.), oh please,
  like when he fought the enemies of Israel
  at the mountain of Perazim
  and the valley of Gibeon.
  His amazing plans shall come to pass
  and (he) shall do unspeakable things.” (Source: Kupsabiny Back Translation)
• Newari: “The LORD will stand like he stood on Mount Perazim,
  [and] he will come like he came to the valley of Gibeon,
  to carry out his strange work.” (Source: Newari Back Translation)
• Hiligaynon: “The truth is, the LORD will-attack like what he did to Mount Perazim and in the Plains/Valley of Gibeon. He will-do the thing which his people is- not -expecting.” (Source: Hiligaynon Back Translation)
• English: “Yahweh will come and cause you to be defeated;
  he will do to you like he did to the army of Philistia at Perizim Mountain
  and like he did to the Amor people-group at Gibeon Valley.
  What he will do will be very strange and unusual.” (Source: Translation for Translators)