Following are a number of back-translations as well as a sample translation for translators of Psalm 55:21:

• Chichewa Contempary Chichewa translation, 2002/2016:
  “His words are smooth more than butter
  however war is in his heart;
  his words are very smooth more than oil,
  but those words are swords that are drawn.” (Source: Mawu a Mulungu mu Chichewa Chalero Back Translation)
• Newari:
  “His voice is
  as smooth as ghee,
  but his heart is
  full of discord,
  his words seem as soft as ghee,
  [but they] are sharper than a sword.” (Source: Newari Back Translation)
• Hiligaynon:
  “He (is) gentle and good in-speech/well spoken
  but bears-a-grudge in his heart,
  and what he speaks can- indeed/[surprise particle] -wound like a sharp sword.” (Source: Hiligaynon Back Translation)
• Laarim:
  “His voice is like oil for smearing,
  but instead, he hates people in his heart,
  his words are like the oil for smearing,
  but instead they are like sharp knife.” (Source: Laarim Back Translation)
• Nyakyusa-Ngonde (back-translation into Swahili):
  “Maneno yake matamu kuzidi siagi,
  lakini mawazo yake yamejaa vita.
  Usemi wake wa upole kuzidi mafuta,
  lakini umekata kama vile upanga mkali.” (Source: Nyakyusa Back Translation)
• English:
  “What he said was easy to listen to, like butter is easy to swallow,
  but in his inner being he hated people;
  his words were soothing like olive oil,
  but they hurt people like sharp swords do.” (Source: Translation for Translators)