Following are a number of back-translations as well as a sample translation for translators of Proverbs 18:17:

• Kupsabiny: “Words of a person who defends himself with the mouth (words) appear to be true,
  only upto when a person has come with a complaint/objection.” (Source: Kupsabiny Back Translation)
• Newari: “When there is a law case,
  the first man to speak seems right
  until the opponent comes and asks a question.” (Source: Newari Back Translation)
• Hiligaynon: “The first who witnesses in the court seems the right-one already, until his opponent will- now -investigate him.” (Source: Hiligaynon Back Translation)
• Kankanaey: “What the first-one who speaks says in court (lit. place-/occasion-of-judging), it seems correct if his-opponent/enemy has not-yet spoken.” (Source: Kankanaey Back Translation)
• English: “The first person to present his case in court seems right,
  but when someone else/his opponent begins to ask him questions, it may become clear that what he said was not true.” (Source: Translation for Translators)