Add language option to server.properties (#2531)

This allows to save the language without rewriting pocketmine.yml. Since this is a "standard" config option (something that the user might want to directly modify) it's reasonable to put it in server.properties. pocketmine.yml is generally reserved for more advanced configuration options.

src/pocketmine/wizard/SetupWizard.php

@@ -66,6 +66,10 @@ class SetupWizard{
 			}
 		}while($lang === null);
 
+		$config = new Config(\pocketmine\DATA . "server.properties", Config::PROPERTIES);
+		$config->set("language", $lang);
+		$config->save();
+
 		$this->lang = new BaseLang($lang);
 
 		$this->message($this->lang->get("language_has_been_selected"));