Fix date picker not properly localized (#330)

* Fix language in date picker

Vuetify allows custom-named locales,
but the date-picker really only works with BCP 47 language tag

* Save lang at proper time + Update Vuetify lang on the fly

frontend/src/components/Admin/General/HomePageSettings.vue

@@ -211,7 +211,6 @@ export default {
 
   methods: {
     writeLang(val) {
-      this.$store.commit("setLang", val);
       this.settings.language = val;
     },
     deleteCategoryfromDatabase(category) {
@@ -225,7 +224,9 @@ export default {
     },
     async saveSettings() {
       await api.siteSettings.update(this.settings);
-      this.$store.commit("setLang", this.settings.language);
+      this.$store.dispatch("setLang", { 
+        currentVueComponent: this, 
+        language: this.settings.language });
       this.getOptions();
     },
   },

frontend/src/components/UI/LanguageMenu.vue

@@ -47,7 +47,7 @@ export default {
       items: [
         {
           name: "English",
-          value: "en",
+          value: "en-US",
         },
       ],
     };
@@ -72,7 +72,9 @@ export default {
       if (this.siteSettings) {
         this.$emit(SELECT_EVENT, selectedLanguage);
       } else {
-        this.$store.commit("setLang", selectedLanguage);
+        this.$store.dispatch("setLang", { 
+          currentVueComponent: this, 
+          language: selectedLanguage });
       }
     },
   },