Sorry, that is an oversight on my part. The dates are not translatable. I made the necessary changes for the next update to be released in a couple weeks. If you are able to make a minor change to a PHP file you could apply the change now. Here’s the file you need to edit:
return gmdate( $format, $nextUDay );
return date_i18n( $format, strtotime( gmdate( 'c', $nextUDay ) ) );
This runs the date thru the core WP function which handles the translation of dates.
Hope this helps!