requestLanguages(requestedLangs) {
    const requests = [
      super.requestLanguages(requestedLangs)
    ].concat(
      Array.from(
        this.delegates.values(),
        delegate => delegate.requestLanguages(requestedLangs)
      )
    );

    return Promise.all(requests).then(
      () => this.translateDocument()
    );
  }
Trigger the language negotation process for this `DocumentLocalization`
and any `DOMLocalization` objects which it can delegate to.

Returns a promise which resolves to an array of arrays of negotiated
languages for each `Localization` available in the current document.

```javascript
document.l10n.requestLanguages(['de-DE', 'de', 'en-US']);
```

@param   {Array<string>} requestedLangs - array of requested languages
@returns {Promise<Array<Array<string>>>}