Following are a number of back-translations as well as a sample translation for translators of Isaiah 29:15:

• Kupsabiny: “The people who hide their plans from God shall suffer the consequences.
  Those people hide the sin they are doing.
  They are saying,
  ‘There is no one who sees us
  and there is no one who knows us.’” (Source: Kupsabiny Back Translation)
• Newari: “Alas to you,
  any who try to hide their plans from the LORD,
  any who do their work in a dark place,
  thinking, "Will anyone see us? Will anyone find out?"” (Source: Newari Back Translation)
• Hiligaynon: “You (plur.) are-to-be-pitied who hide your (plur.) plans from the LORD. You (plur.) do your (plur.) works in darkness, and you (plur.) say, ‘No one sees or knows what we (excl.) are-doing.’” (Source: Hiligaynon Back Translation)
• English: “Terrible things will happen to those who try to conceal from me, Yahweh,
  the evil things that they plan to do;
  they do those things in the darkness
  and they think, ‘Yahweh certainly cannot see us;
  he cannot know what we are doing!’” (Source: Translation for Translators)