[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.py

@@ -203,6 +203,7 @@ about: dict[str, str | bool] = {
 categories: list[str] = ["general", "web"]
 paging: bool = True
 time_range_support: bool = True
+language_support = True
 safesearch: bool = True
 """DDG-lite: user can't select but the results are filtered."""