Reads the .po file, but only if it has not been loaded or has changed

@returns {Promise}|
async function() {
      if (!this.__mtime) {
        return this.read(); 
      }
      var poFile = this.getPoFilename();
      let stat = await qx.tool.compiler.files.Utils.safeStat(poFile);
      if (stat && this.__mtime == stat.mtime) {
        return undefined;
      }
      return this.read();
    }