Add support for en-GB (#629)

* Add support for en-GB

Allows for British date format (native in vue-i18n)
+ potential spelling adaptations

* Rename default English locale to American English

frontend/src/plugins/vuetify.js

@@ -43,6 +43,7 @@ const vuetify = new Vuetify({
     locales: {
       "de-DE": de,
       "en-US": en,
+      "en-GB": en,
       "es-ES": es,
       "fr-FR": fr,
       "it-IT": it,

frontend/src/store/modules/language.js

@@ -2,9 +2,13 @@
 const state = {
   allLangs: [
     {
-      name: "English",
+      name: "American English",
       value: "en-US",
     },
+    {
+      name: "British English",
+      value: "en-GB",
+    },
     {
       name: "Deutsch (German)",
       value: "de-DE",