Update cache checking logic in core/includes/langauge.inc to use config system

Child ticket of #1775842: [meta] Convert all variables to state and/or config systems.

There is a call to variable_get('cache') in language.inc that was missed when this variable was moved to the config system:

core/includes/language.inc
475 $cache = !isset($method['cache']) || $user->isAuthenticated() || $method['cache'] == variable_get('cache', 0);

For an example that was converted see:
7.x: http://drupalcode.org/project/drupal.git/blame/ace62d67ea77fdfe0e72a2eb3...
8.x: http://drupalcode.org/project/drupal.git/blame/7e1a0ca0bba94a66a60094b0d...