Fix switch to default language from site url

src/modules/slugs/early_hooks.php

@@ -3,7 +3,9 @@
 add_filter( 'qtranslate_language_detect_redirect', 'qtranxf_slugs_language_detect_redirect', 600, 3 );
 
 /**
-* Disables all redirections except when site url is requested, if default language is hidden. In this case url is redirected to default language site url.
+* Disables default redirection when language is not detectable from url (e.g. default language url with hide_default_language activated).
+* Exception is made for site_url(), for which language detection feature is preserved.
+* 
 *
 * @see qtranxf_check_url_maybe_redirect
 * @param string $url_lang proposed target URL for the active language to redirect to.
@@ -14,23 +16,16 @@
 */
 function qtranxf_slugs_language_detect_redirect($url_lang, $url_orig, $url_info): string {
    global $q_config;
-
-   if ( ( site_url().'/' === $url_orig && $q_config['hide_default_language'] ) || empty( $url_info['lang_url'] ) ){
-
-      $url_res = qtranxf_convertURL( $url_orig, $q_config['default_language'],false,true );
-
-      if ( qtranxf_detect_language_front( $url_info ) != $q_config['default_language'] ) {
-
-          if ( $url_info['cookie_front_or_admin_found'] )
-            qtranxf_set_language_cookie( $q_config['default_language'] );
-
-          if ( $url_res == $url_orig )
-            $q_config['url_info'] = qtranxf_url_set_language( $urlinfo, $q_config['default_language'], $showLanguage );
-
-      }
-
-   } else
-      $url_res = $url_orig;
-
-   return $url_res;
+   /* Make sure urls with no lang info are treated as default language, unless it's site_url */
+   if ( untrailingslashit( site_url() ) != untrailingslashit( $url_orig ) && empty( $url_info['lang_url'] ) ){
+      return qtranxf_convertURL( $url_orig, $q_config['default_language'], false, true );
+   /* The following fixes the case when the browser removes language marker from default language site url (caching?) passing directly _$SERVER['REQUEST_URI'] with language info removed.
+    * In this case is not possible to switch to default language from site url.
+    * Application of this hack is narrowed down to the cases where referer is site url in current language, to preserve language detection feature as much as possible.
+    * @TODO: check if a cleaner fix is applicable. */
+   } else if ( $q_config['hide_default_language'] && untrailingslashit( site_url() ) == untrailingslashit( $url_orig ) && untrailingslashit($url_lang) === untrailingslashit( wp_get_raw_referer() ) ) {
+      return qtranxf_convertURL( $url_orig, $q_config['default_language'], false, true );
+   }
+   /* All other cases follow default behaviour */
+      return $url_lang;
 }