I've been using the multiple language functioning in Survey123, and the preview of the survey automatically carries the default language over to any untranslated aspects of the survey. The default language below is in English and the translated language is Spanish. Note the submit button in this image:

But the published version of the survey fills any untranslated aspects in with "-"s instead of the default language. See the same surveys published version:

Note the submit button is just a "-". All to say the preview does not accurately preview what the published version will be like. It is more useful for the default language to be automatically added than "-"s.