code: function getUserDefinedLocale() {
  	const locale = args['locale'];
  	if (locale) {
  		return Promise.resolve(locale.toLowerCase());
  	}
  
  	const localeConfig = path.join(userDataPath, 'User', 'locale.json');
  	return bootstrap.readFile(localeConfig).then(content => {
  		content = stripComments(content);
  		try {
  			const value = JSON.parse(content).locale;
  			return value && typeof value === 'string' ? value.toLowerCase() : undefined;
  		} catch (e) {
  			return undefined;
  		}
  	}, () => {
  		return undefined;
  	});
  }
docs: Language tags are case insensitive however an amd loader is case sensitive To make this work on case preserving & insensitive FS we do the following: the language bundles have lower case language tags and we always lower case the locale we receive from the user or OS. 
  @returns {Promise<string>}