Fixed language detection for no regional variant #175

This commit refines the approach to extracting language codes from the `Accept-Language` header. 

By employing a unified regex pattern, the code now efficiently captures both simple language codes and those with regional variants. 

The language code extraction logic now also splits on both hyphens and underscores, defaulting to the primary language code in cases of regional variants, and always converts the result to lowercase for consistent processing.

examples/multi-page/src/routes/+layout.server.js

@@ -8,9 +8,21 @@ export const load = async ({ url, cookies, request }) => {
   let locale = (cookies.get('lang') || '').toLowerCase();
 
   // Get user preferred locale
-  if (!locale) {
-    locale = `${`${request.headers.get('accept-language')}`.match(/[a-zA-Z]+?(?=-|_|,|;)/)}`.toLowerCase();
-  }
+ 	if (!locale) {
+		// If no cookie is set, try to determine the locale from the 'Accept-Language' header
+		const acceptLanguageHeader = request.headers.get('accept-language') || '';
+		// Attempt to match the language code with optional region code
+		let match = acceptLanguageHeader.match(/^[a-z]+(?=[-_])/i);
+
+		// If no match is found, try to match just the language code
+		if (!match) {
+			match = acceptLanguageHeader.match(/^[a-z]+/i);
+		}
+
+		// If a match is found, use it as the locale, otherwise fall back to the default locale
+		locale = match ? match[0].toLowerCase() : defaultLocale;
+	}
+
 
   // Get defined locales
   const supportedLocales = locales.get().map((l) => l.toLowerCase());
@@ -26,4 +38,4 @@ export const load = async ({ url, cookies, request }) => {
     i18n: { locale, route: pathname },
     translations: translations.get(), // `translations` on server contain all translations loaded by different clients
   };
-};
+};