This notice showed up in the Admin panel sometime today/yesterday and I can’t find what the issue is. I’ve fully updated everything and followed the hotlink “check your text customizations” leads to /customize/site_texts?outdated=true and I went through every single language option with Only show outdated/invalid selected and nothing shows up in any of them.
I don’t believe this is actually affecting anything but I can’t get rid of the notice
The same thing happens to me. Invalid texts do not appear when I go to text customizations.
Same issue here
Must have been from one of the main updates then and not something I/we broke
Thanks for reporting this - we’re looking into the problem and should have a fix soon
It definitely was related to the last update I was reading through the changes and it’s exactly this:
Continuing the discussion from 3.1.0.beta6: Threads in Chat, Image Grid, Export Chat Messages, and more:
It’s just not quite working right/is hiding something
Thanks for the report! 
I noticed the same thing was happening here on Meta, and I found an issue where we don’t consider the case where there is an old translation for a translation key that has later been renamed or removed from Discourse core. These cases were incorrectly being marked as having invalid interpolation keys.
I have opened a pull request with a fix for this here.
It’s gone now (for me at least) 