[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/duckduckgo_extra.py

@@ -28,6 +28,7 @@ about = {
     "require_api_key": False,
     "results": "JSON (site requires js to get images)",
 }
+language_support = True
 
 # engine dependent config
 categories = []