✨(frontend) add language utility for "locale"

- Adds a helper for working with locales
- More details in their annotations
- Unnecessary, if in the future, the backend uses
  the same locales as the keys in the translations (ISO 639-1)

src/frontend/apps/impress/src/features/language/utils/locale.ts

@@ -0,0 +1,49 @@
+/**
+ * @param {string} locale - The locale string, which can be in formats like de-DE, de-CH, de_DE, de-de, de_de, or de.
+ * @returns {string} The regionless ISO 639-1 code. => de
+ */
+export function convertLocaleToISO639_1(locale: string): string {
+  // Split the locale string by either a hyphen (-) or an underscore (_)
+  return locale.split(/[-_]/)[0].toLowerCase();
+}
+
+/**
+ * Filters the available locales to find those that match the search criteria.
+ *
+ * @param {readonly string[]} localesAvailable - The list of available locale strings to match against.
+ * @param {readonly string[]} localesToSearch - The list of locale strings to search for.
+ * @returns {string[]} Filtered list of localesAvailable that match localesToSearch, ordered by closest match.
+ */
+export function getMatchingLocales(
+  localesAvailable: readonly string[],
+  localesToSearch: readonly string[],
+): string[] {
+  const matchingLocales: string[] = [];
+  for (const localeToSearch of localesToSearch) {
+    // Exact match (case-insensitive)
+    const exactMatches = localesAvailable.filter(
+      (localeAvailable) =>
+        localeAvailable.toLowerCase() === localeToSearch.toLowerCase(),
+    );
+    if (exactMatches.length > 0) {
+      for (const matchingLocale of exactMatches) {
+        if (!matchingLocales.includes(matchingLocale)) {
+          matchingLocales.push(matchingLocale);
+        }
+      }
+      continue;
+    }
+    // Soft match (ISO639-1 code)
+    const softMatches = localesAvailable.filter(
+      (localeAvailable) =>
+        convertLocaleToISO639_1(localeAvailable) ===
+        convertLocaleToISO639_1(localeToSearch),
+    );
+    for (const matchingLocale of softMatches) {
+      if (!matchingLocales.includes(matchingLocale)) {
+        matchingLocales.push(matchingLocale);
+      }
+    }
+  }
+  return matchingLocales;
+}