I open this as a separate topic to focus on one specific but important choice: the date format used if the user did not set their language in the user preferences.
The current default, suggested in the other thread, is to use the system date format. In some cases, that doesn’t seem to happen correctly.
An alternative proposal would be using the ISO 8601 format: %Y-%m-%d
The benefit of this proposal is that it is really obvious that there is no default.
Currently, if people enter values where %d is less than or equal to 12, the day and month are swapped and the user may not realize something bad happened.