fix: localize date formatting

app.js

@@ -13,6 +13,8 @@ const POPULAR_NAMES = [
 
 const timezoneByName = new Map(SORTED_TIMEZONE_DATA.map((entry) => [entry.name, entry]));
 
+const LOCALE = 'de-DE';
+
 function normalizeText(value) {
   return value
     .toLowerCase()
@@ -80,7 +82,7 @@ function resolveInitialTheme() {
 }
 
 function buildLocalLabel(timeZone) {
-  const formatter = Intl.DateTimeFormat(undefined, {
+  const formatter = Intl.DateTimeFormat(LOCALE, {
     timeZone,
     timeZoneName: 'longGeneric'
   });
@@ -95,7 +97,7 @@ function buildLocalLabel(timeZone) {
 }
 
 function formatClockValue(date, timeZone) {
-  return new Intl.DateTimeFormat(undefined, {
+  return new Intl.DateTimeFormat(LOCALE, {
     timeZone,
     hour: '2-digit',
     minute: '2-digit',
@@ -105,7 +107,7 @@ function formatClockValue(date, timeZone) {
 }
 
 function formatDateValue(date, timeZone) {
-  return new Intl.DateTimeFormat(undefined, {
+  return new Intl.DateTimeFormat(LOCALE, {
     timeZone,
     weekday: 'long',
     year: 'numeric',