Add the language parameter to your Widget code
UserVoice.push(["set", "locale", "de"]);...to your Widget code.
So the "de" would be whatever language your website passed to us for that user. This needs to be added to your Widget code as its own command in the script block.
What this will translate...
This will translate the Widget UI text. It will not translate the ideas, custom fields or Knowledge Base articles. If a word or phrase of the Widget UI has not been translated on our translation portal, it will default to English.
To learn more about our different multi-language options, check out this article.