BasicsPage: fix "show language selector" default

Since some boolean settings might be stored as string "0" or "1", the previous system no longer works, and it always sets the switch to true. The "no setting" check has been changed to reference `undefined`, so now the switch will only be defaulted to `true` if the setting truly hasn't been set.

Fixes #2574

js/src/admin/components/BasicsPage.js

@@ -61,7 +61,7 @@ export default class BasicsPage extends Page {
       }
     });
 
-    if (typeof this.values.show_language_selector() !== 'number') this.values.show_language_selector(1);
+    if (this.values.show_language_selector() === undefined) this.values.show_language_selector(1);
   }
 
   view() {