Improve i18n language matching (#5939)

src/i18n.js

@@ -75,13 +75,22 @@ export function currentLocale() {
         if (locale in messages) {
             return locale;
         }
-        // some locales are further specified such as "en-US".
+        // If the locale is a 2-letter code, we can try to find a regional variant
+        // e.g. "fr" may not be in the messages, but "fr-FR" is
+        if (locale.length === 2) {
+            const regionalLocale = `${locale}-${locale.toUpperCase()}`;
+            if (regionalLocale in messages) {
+                return regionalLocale;
+            }
+        } else {
+            // Some locales are further specified such as "en-US".
             // If we only have a generic locale for this, we can use it too
-        const genericLocale = locale.split("-")[0];
+            const genericLocale = locale.slice(0, 2);
             if (genericLocale in messages) {
                 return genericLocale;
             }
         }
+    }
     return "en";
 }