Following are a number of back-translations as well as a sample translation for translators of 1 Chronicles 12:18:

• Kupsabiny: “Then the Spirit came to/on Amasai who would later become the leader of those thirty fierce men. Then Amasai said,
  We are yours, oh David,
  and we are together with you, oh son of Jesse!
  Please, may you receive peace,
  and your helpers prosper,
  because it is God who comes to your aid.’
  Then, David welcomed those people and gave them to lead his soldiers.” (Source: Kupsabiny Back Translation)
• Newari: “At that time, the Spirit of God gave words of encouragement to Amasai, leader of the thirty. Then he spoke like this,
  "Oh David, son of Jessie,
  we are for you.
  May you succeed!
  May those who help you
  also succeed,
  May God be with you!"
  And David welcomed them. He made them leaders of his army.” (Source: Newari Back Translation)
• Hiligaynon: “Then the Spirit empowered Amasai who later on became a leader of 30 brave soldiers, and he said,
  ‘We (incl.) are yours, O David the child of Jesse! May you (sing.) be-successful/victorious and those who-help-you (sing.), for your (sing.) God is-helping you (sing.).’ So David received them and made (them) officers of his soldiers.” (Source: Hiligaynon Back Translation)
• English: “Then God’s Spirit came upon Amasai, who was another leader of the thirty greatest warriors, and he said,
  ‘David, we want to be with you;
  you who are the son of Jesse, we will join you.
  We know that things will go very well for you and for those who are with you,
  because your God is helping you.’” (Source: Translation for Translators)