I own a Mi 9 (cepheus) and recently updated to MIUI 12 Stable (MIUI 12.0.1.0.QFACNXM). The language of my phone is set to "English (United Kingdom)".
Before the upgrade to MIUI 12 I was on MIUI 11 Stable and the date format was displayed correctly for me like this: DD.MM.YYYY.
Now, the date...