formatValue(id, args) {
    return this.formatValues([id, args]).then(
      ([val]) => val
    );
  }
Retrieve the translation corresponding to the `id` identifier.

If passed, `args` is a simple hash object with a list of variables that
will be interpolated in the value of the translation.

    localization.formatValue(
      'hello', { who: 'world' }
    ).then(console.log);

    // 'Hello, world!'

Returns a Promise resolving to the translation string.

Use this sparingly for one-off messages which don't need to be
retranslated when the user changes their language preferences, e.g. in
notifications.

@param   {string}  id     - Identifier of the translation to format
@param   {Object}  [args] - Optional external arguments
@returns {Promise<string>}