lang API: Fixup dd83225#diff-9ab5e4fd4c9b654a7843bf0b989f8c4653595e31…

…aa0875417172955a6a215464 for the undocumented private variable usage to overwrite lang constants via user theme

I think backend allowance was just an overseen mistake while this commit was a draft commit for splitting up backend and frontend themes for Serendipity 2.0.
It makes no sense to have this anywhere other than for individual user frontend themes. The less it runs through, the less possible regressions it has.
It was done in the early days to allow overwriting certain core constants in themes for frontend templates, e.g. POWERED_BY.

include/lang.inc.php

@@ -14,23 +14,20 @@
     $charset = serendipity_getCharset();
 
     // The following variable can be set in serendipity_config_local.inc.php to force your templates being able to use language override includes
-    if (isset($serendipity['useTemplateLanguage']) && $serendipity['useTemplateLanguage'] === true) {
+    // An un-promoted private variable set, to first serve template based serendipity lang constants that have higher priority.
+    // We do not allow this for backend constants and both, the fallback "default" and the standard theme (currently "pure") !
+    if (isset($serendipity['useTemplateLanguage']) && $serendipity['useTemplateLanguage'] === true && !empty($serendipity['template'])
+    && !defined('IN_serendipity_admin') && !in_array($serendipity['template'], ['default', 'default-php', $serendipity['defaultTemplate']])) {
         if (defined('S9Y_DATA_PATH')) {
             @include_once (S9Y_DATA_PATH . 'templates/' . $serendipity['template'] . '/' . $charset .  'lang_' . $serendipity['lang'] . '.inc.php');
             @include_once (S9Y_DATA_PATH . 'templates/' . $serendipity['template'] . '/lang_en.inc.php');
-
-            @include_once (S9Y_DATA_PATH . 'templates/' . $serendipity['template_backend'] . '/' . $charset .  'lang_' . $serendipity['lang'] . '.inc.php');
-            @include_once (S9Y_DATA_PATH . 'templates/' . $serendipity['template_backend'] . '/lang_en.inc.php');
         } else {
             @include_once (S9Y_INCLUDE_PATH . 'templates/' . $serendipity['template'] . '/' . $charset .  'lang_' . $serendipity['lang'] . '.inc.php');
             @include_once (S9Y_INCLUDE_PATH . 'templates/' . $serendipity['template'] . '/lang_en.inc.php');
-
-            @include_once (S9Y_INCLUDE_PATH . 'templates/' . $serendipity['template_backend'] . '/' . $charset .  'lang_' . $serendipity['lang'] . '.inc.php');
-            @include_once (S9Y_INCLUDE_PATH . 'templates/' . $serendipity['template_backend'] . '/lang_en.inc.php');
         }
     }
 
-    // Try and include preferred language from the configurated setting
+    // Try and include preferred language from the configured setting
 
     if (@include(S9Y_INCLUDE_PATH . 'lang/' . $charset . 'serendipity_lang_'. $serendipity['lang'] .'.inc.php') ) {
         // Only here can we truly say the language is loaded
@@ -42,9 +39,9 @@
         // If no config file is loaded, language includes are not available.
         // Now include one. Try to auto-guess the language by looking up the HTTP_ACCEPT_LANGUAGE.
         serendipity_detectLang(true);
-    }  //endif
+    }
 
-    // Do fallback to english
+    // Do fall back to english
     if (IS_installed === false || (defined('IS_up2date') && IS_up2date === false)) {
         @include_once(S9Y_INCLUDE_PATH . 'lang/' . $charset . 'serendipity_lang_en.inc.php');
     }