add interval for external update-check; add parameter 'force' to Froxlor.checkUpdate() API call; fix session/language update when changing language

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

customer_index.php

@@ -215,6 +215,7 @@ if ($page == 'overview') {
 					'id' => $userinfo['customerid'],
 					'def_language' => $def_language
 				])->update();
+				CurrentUser::setField('language', $def_language);
 			} catch (Exception $e) {
 				Response::dynamicError($e->getMessage());
 			}