More generic slug generation

Co-authored-by: Wouter de Jong <wouterj@users.noreply.github.com>

Author: jaapio

packages/guides-restructured-text/src/RestructuredText/Directives/ConfigurationBlockDirective.php

@@ -60,7 +60,7 @@ protected function processSub(
             $language = $child->getLanguage();
             assert($language !== null);
 
-            $label = $this->languageLabels[$language] ?? ucfirst(str_replace('-', ' ', $language));
+            $label = $this->languageLabels[$language] ?? $this->slugger->slug($language, ' ')->title()->toString();
 
             $tabs[] = new ConfigurationTab(
                 $label,