[fix] set language_support for engines with languages in traits (#6258)

In the past, the engine option ``language_support`` was not consistently
maintained; with this patch, a ValueError is now thrown if an engine has
languages in its traits but language_support is not set to True.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/engines/startpage.py

@@ -131,6 +131,7 @@ max_page = 18
 """Tested 18 pages maximum (argument ``page``), to be save max is set to 20."""
 
 time_range_support = True
+language_support = True
 safesearch = True
 
 time_range_dict = {"day": "d", "week": "w", "month": "m", "year": "y"}