About [[Pywikibot:Revertbot-revert/en]]
Needs date and time parametes separated. See: Separate times from dates in sentences.
As far as I can see, %(timestamp)s is date/time in ISO format ('2006-01-26T12:05:04Z'), so this should not really be an issue. As long as we don't do proper date formatting, I don't think it makes sense to adapt the messages to allow that.
Well, ISO dates are completely unsuited for translated messages. What to do?
Feel free to open a feature request on phab to improve date formatting in revertbot, but it's unlikely to be implemented soon. Doing date formatting *correctly* means we actually have to implement a way to format localized dates and times, which (as far as I know) we currently don't have.
I'm not sure though why you feel ISO dates 'are completely unsuited'. The goal of the message is to transmit four bits of information ('we reverted', revision X, which was created by Y at date/time Z) and, even if the translated message is imperfect, this message can be transmitted.
ISO dates are understandable for people with an anglosaxon background - be it learned or grown up in. For most other cultures they are either completely unreadable or at least misleading. So that is not a little imperfection that we are talking about here, they are unsuited for i18n.
Realistically, there is no way to implement this in pywikibot at the moment. Either we implement it ourselves (not realistic), or we hook into ICU (which is possible, has issues for windows users). I created phab:T102174 to track this; please continue the discussion there.