`mergeLocalizationJSON()` doesn't apply translations to item collections

[andrewtelnov]

Here is the example that doesn't work

const plainJSON = { elements: [{
    type: "slider",
    name: "q1",
    customLabels: [1, 5]
  }]};
const enJSON = { elements: [{
    type: "slider",
    name: "q1",
    customLabels: [{ text: "Very bad" }, { text: "Very good" }]
  }]};
const deJSON = { 
  locale: "de",
  elements: [{
    type: "slider",
    name: "q1",
    customLabels: [{ text: "Sehr schlecht"}, { text: "Sehr gut" }]
  }]};

const survey = new SurveyModel(plainJSON);
survey.mergeLocalizationJSON(enJSON);
survey.mergeLocalizationJSON(deJSON);

As result the custom labels will not contain the English text and/or German text