chore(i18n): update German language localisation (#1265)

* Add common German date format.
* Fix translation of read time prompt to a more correct word.

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Co-authored-by: Cotes Chung <11371340+cotes2020@users.noreply.github.com>

Author: jensboe

_data/locales/de-DE.yml

@@ -58,7 +58,7 @@ post:
   pageview_measure: Aufrufe
   read_time:
     unit: Minuten
-    prompt: lesen
+    prompt: Lesezeit
   relate_posts: Weiterlesen
   share: Teilen
   button:
@@ -70,6 +70,13 @@ post:
       title: Link kopieren
       succeed: Link erfolgreich kopiert!
 
+# Date time format.
+# See: <http://strftime.net/>, <https://day.js.org/docs/en/display/format>
+df:
+  post:
+    strftime: "%d.%m.%Y"
+    dayjs: "DD.MM.YYYY"
+    
 # categories page
 categories:
   category_measure: