Add string for future app language toggle preference

Feature won't be added until A13 and/or the Jetpack library is stabler, but adding the string first so translations flow in.

app/src/main/res/values/strings.xml

@@ -179,6 +179,7 @@
 
     <string name="pref_confirm_exit">Confirm exit</string>
     <string name="pref_manage_notifications">Manage notifications</string>
+    <string name="pref_app_language">App language</string>
 
     <string name="pref_category_security">Security</string>
     <string name="lock_with_biometrics">Require unlock</string>