getPropKeyForLang = function(propKey, lang) {
  if (!lang || lang == "en-US") return propKey;
  else if (lang == "zh-CN") return propKey + "_zhCN";
  else throw new Error("Not implemented yet");
}
Get the property key appended with a language code except en-US.
@param {String} propKey - the property key
@param {String} lang - the ISO 639-1 standard language code
@returns the property key for lang