feat: add German language support (#207)

* feat: add German language support

* fix: translate other locale names to German

* chore: integrate recommendations from the PR review

* i18n: add translation recommendations from the PR

* Fix translations

---------

Co-authored-by: Christian Schuller <christianschuller.biz@gmail.com>
Co-authored-by: Sebastien Castiel <sebastien@castiel.me>

messages/es.json

@@ -297,7 +297,8 @@
     "en-US": "English",
     "fi": "Suomi",
     "fr-FR": "Français",
-    "es": "Español"
+    "es": "Español",
+    "de-DE": "Deutsch"
   },
   "Share": {
     "title": "Compartir",