The language_support variable is set to True by default,
and set to False in only 5 engines.
Except the documentation and the /config URL, this variable is not used.
This commit remove the variable definition in the engines, and
set value according to supported_languages length: False when the length is 0,
True otherwise.
Close#2485
Searching for english is now giving all pages results.
Instead searching for a specific language different from english
is correctly returning only translated pages for the selected language.
