async formatValue(id, args) {
    const [val] = await this.formatValues([{ id, args }]);
    return 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.

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.

@example
```js
docL10n.formatValue(
  'hello', { who: 'world' }
).then(console.log);

// 'Hello, world!'
```

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