translateDocument() {
    const localizations = [this, ...this.delegates.values()];
    return Promise.all(
      localizations.map(
        l10n => l10n.translateRoots()
      )
    );
  }
Triggers translation of all roots associated with this
`DocumentLocalization` and any `DOMLocalization` objects which it can
delegate to.

Returns a `Promise` which is resolved once all translations are
completed.

@returns {Promise}