singleton, global vars and dontdestroyonload

Hello,

I have troubles carrying on my global vars trough scenes. I change the language setting in my menu ingame, then i launch a scene via a new game button and the setting is gone.

Here is my GlobalVar class:

 using UnityEngine;
 using System.Collections;
 
 public enum GameDifficulty
 {
     Easy,
     Medium,
     Hard
 }
 ;
 
 public enum Language
 {
     english,
     french
 }
 ;
 
 public class GlobalVars : Singleton<GlobalVars> {
     protected GlobalVars () {} // guarantee this will be always a singleton only - can't use the constructor!
     
     public GameDifficulty difficulty = GameDifficulty.Medium;
     public Language language = Language.english;
 
     void awake()
     {
         DontDestroyOnLoad(gameObject);
     }
 }

I also have a script in my project which contain a singleton class (used for an audio manager). Maybe it have something to do with this. Both are codes i didn't wrote myself and I'm so lost! Here it is:

 using UnityEngine;
 using System.Collections;
 
 public class Singleton<T> : MonoBehaviour where T : MonoBehaviour {
     protected static T instance;
     
     //Returns the instance of this singleton
     public static T Instance {
         get {
             if (instance == null) {
                 instance = (T)FindObjectOfType(typeof(T));
                 if (instance == null) {
                     GameObject container = new GameObject();
                     container.name = typeof(T)+"Container";
                     instance = (T)container.AddComponent(typeof(T));
                 }
             }
             return instance;
         }
     }
 }

Can you please tell me where i'm wrong?