site generates broken multiplart/alternative emails, no plain text just error message

since a few weeks ago, lots of the mails to the bb not-groups arrive with
a text/plain body just consisting of this error message

“translation missing: en.user_notifications.user_watching_category_or_tag.text_body_template
To unsubscribe from these emails, [click here](…”

as they’re tagged as multipart/alternative, looking at the other, text/html part, it seems some of the submitter’s content is present.