I’ve seen this too - it’s still happening in master. If I recall correctly my conclusion was that a duplicate entry in the locale file is causing this. But debugging i18n issues is kinda annoying at the moment and I didn’t have enough time to verify my bugfix. (@sam What’s your take on this: Always enable localization fallback)
https://github.com/discourse/discourse/blob/master/config/locales/server.en.yml#L23-L33
https://github.com/discourse/discourse/blob/master/config/locales/server.en.yml#L389-L460
I guess they should be merged. Currently Transifex sees only the second usage of datetime. If those keys are merged it will make the month_names translatable.