Add Slovenian language option to dropdown

Author: louis-e

src/gui/index.html

@@ -209,6 +209,7 @@ <h2 data-localize="customization_settings">Customization Settings</h2>
               <option value="ko">한국어</option>
               <option value="pt-BR">Português (Brasil)</option>
               <option value="pl">Polski</option>
+              <option value="sl">Slovenščina</option>
               <option value="lv">Latviešu</option>
               <option value="sv">Svenska</option>
               <option value="ar">العربية</option>