It happens so infrequently that I am not yet able to find a pattern or common point for this issue. I will continue to look for such patterns if or when it happens again. In the past when we had such problems (it was a more common occurrence a couple of years ago), we gave up on translation substitutions and would include text in both languages in the generated emails. But this kind of language substitution fail has mostly disappeared as CB and related plugins continue to evolve. Since we launched our revised website in June of this year using lots more CB functionality, this is the first language substitution issue we have had (and there have been A LOT of language substitution based emails generated since that time!). I don't think you need to devote a lot of time trying to "fix" this just yet. I mostly wanted to let you know about it. I will follow up this thread with an update if it happens again, hopefully with more information related to the possible cause.