Since Wikidata's early days it had been possible to use Wikidata's interface in languages other than English. However, dates have so far been half-localized by substituting the month name with the month name in the target language without localizing the date format string.
This results in major inconvenience to users of languages where the date format string is not "d Mmm yyyy" or "Mmm d yyyy". In many cases the partially localized dates make no sense to a native reader.
This task requests that language-specific format strings to be applied when Wikidata displays any date. Until that is implemented, incomplete localizations should be reverted to an international date format (e.g. 2012-10-29) for languages that do not use a "d Mmm yyyy" format string,
For example, per this thread, the Portuguese interface displays "junho 12 1990" for this item which is wrong Portuguese; the correct date format should be "12 de junho de 1990".
In other languages, such as Chinese (all variants), Japanese, and Korean where month names are simply numbers, such a localisation results in a mangled string of numbers which make little sense to the reader, for example "22 五月 2017" which does not make sense to a Chinese reader; the correct format string should be "2017年5月22日".
Also many more date formats per languages should be changed.
Whiteboard: papercut u=dev c=backend p=3