Update index.md

Author: qwertygc

docs/doc/index.md

@@ -9,3 +9,13 @@
 
 - [Français](/doc/fr)
 
+<script>
+const availableLanguages = ['en', 'fr'];
+const defaultLanguage = 'en';
+const browserLanguage = navigator.language.split('-')[0];
+if (availableLanguages.includes(browserLanguage)) {
+  window.location.href = `/${browserLanguage}/`;
+} else {
+  window.location.href = `/${defaultLanguage}/`;
+}
+</script>