let unsupportedLanguages = ["de", "fr"]

let surveyObject = {
      locale: "en",
      pages: [
        {
          name: "page1",
          elements: [
            {
              type: "matrixdropdown",
              choices: [
                {
                  value: 5,
                  text: {
                    default: "5 (the best)",
                    de: "5 (Das beste)",
                    fr: "5 (Le meilleur)"			
                  }
                },
                4,
                3,
                2, {
                  value: 1,
                  text: {
                    default: "1 (the worst)",
                    de: "1 (Das Schlechteste)"
                  }
                }
              ],
              columns: [
                {
                  name: "rate",
                  title: {
                    default: "Rating",
                    de: "Bewertung"
                  }
                }
              ],
              name: "favoriteMovie",
              rows: [
                {
                  value: "moonlight",
                  text: "Moonlight"
                },
                {
                  value: "zootopia",
                  text: {
                    default: "Zootopia",
                    de: "Zoomania"
                  }
                },
                {
                  value: "water",
                  text: "Hell or High Water"
                },
                {
                  value: "manchester",
                  text: "Manchester by the Sea"
                }, {
                  value: "lalaland",
                  text: "La La Land"
                }
              ],
              title: {
                default: "Please rate these movies",
                de: "Bitte bewerten Sie diese Filme"
              }
            }
          ]
        }
      ]
    };

const surveyJson = JSON.stringify(surveyObject, function (key, value) {
  if (unsupportedLanguages.includes(key)) {
    return undefined;
  } else {
    return value;
  }
});

console.log(JSON.parse(surveyJson));