The Greek that is rendered as “faithful” in English is (back-) translated in various ways:

• Toraja-Sa’dan: “honest/straight”
• North Alaskan Inupiatun: “unchangeable”
• Highland Totonac “one who fulfills” (source for this and above: Reiling / Swellengrebel)
• Tsou: “actively following closely” (source: Peng Kuo-Wei)
• Mende: “doesn’t turn this way and that” (source: Rob Koops)
• Sinasina: “following well” (source: Paratext Consultant Notes)
• Enlhet “doesn’t go past his word” (source: Jacob Loewen in The Bible Translator 1969, p. 24ff. )
• Kituba “put one’s heart into it” (source: Donald Deer in The Bible Translator 1973, p. 207ff. )
• Noongar: hkoort-karni or “heart true” (source: Warda-Kwabba Luke-Ang)

See also faith / believe.