When searching OTRS's github for "%Y-%M-%D %T"
it returns only one result and that's on the en_CA.pm file which I don't think is the actual issue.
Still, I did some further searching (using your example tag <OTRS_TICKET_SolutionTimeDestinationDate> as base) and found that the notification tags <OTRS_TICKET_*> are basically getting the parameters from the TicketGet method
which in turn get's all it's escalation parameters
from the TicketEscalationDateCalculation method
That method uses the DateTime module
to output its result string
Finally, it's this DateTime::toString()
method that's outputting the dates in the following format: '%Y-%m-%d %H:%M:%S' which I assume is exactly what you're getting.
So, in conclusion, I guess this is either a missing feature or maybe it's actually like this by design as (at least) that escalation parameter doesn't appear to be getting converted to any of the date formats available on the language files.
Disclaimer: I'm not connected with OTRS development in any way. I just did some code digging so it's possible that I missed something.
Maybe post on bugs.otrs.org
to get some feedback from the developers.