i18n-polyfill fix

The documentation says that you need this code in your app.module.ts to load language files dynamically:
{
  provide: TRANSLATIONS,
  useFactory: (locale) => {
    locale = locale || 'en'; // default to english if no locale provided
    return require(`raw-loader!../locale/messages.${locale}.xlf`);
  },
  deps: [LOCALE_ID]
}

But you have to keep in mind that this will fail if you try to load your default language file.
For example we write our App in German and then generate with ng xi18n a translation.de.xlf file. This default file does
correctly only have a source language and no target language. But without them i18n-polyfill will crash. So you have to
extend the code:

{
  provide: TRANSLATIONS,
  useFactory: (locale) => {
  locale = locale || 'en';
  if (locale === 'de') { // de is our default
    return '';
  } else {
    return require(`raw-loader!../../i18n/translation.${locale}.xlf`);
  }
  },
  deps: [LOCALE_ID]
}

Here you return nothing but an empty string if your app runs with the default language.