Resolved How to set starting locale?

Use the Locale selectors. We already have a player preference one https://docs.unity3d.com/Packages/com.unity.localization@0.9/manual/LocalizationSettings.html
Click the + icon to add one.
You can also create a custom one by inheriting from IStartupLocaleSelector
This is the PlayerPrefOne:

Code (csharp):

1.  
2. using System;
3.  
4. namespace UnityEngine.Localization.Settings
5. {
6.     /// <summary>
7.     /// Uses the Player Prefs to keep track of the last used locale.
8.     /// Whenever the locale is changed, the new Locale is recorded in the Player prefs.
9.     /// </summary>
10.     [Serializable]
11.     public class PlayerPrefLocaleSelector : IStartupLocaleSelector, IInitialize
12.     {
13.         [SerializeField]
14.         string m_PlayerPreferenceKey = "selected-locale";
15.  
16.         /// <summary>
17.         /// The Player Pref key to use.
18.         /// </summary>
19.         public string PlayerPreferenceKey
20.         {
21.             get => m_PlayerPreferenceKey;
22.             set => m_PlayerPreferenceKey = value;
23.         }
24.  
25.         /// <summary>
26.         /// Registers a callback to <see cref="LocalizationSettings.SelectedLocaleChanged"/> in order to save changes made to the Locale.
27.         /// </summary>
28.         /// <param name="settings"></param>
29.         public void PostInitialization(LocalizationSettings settings)
30.         {
31.             if (Application.isPlaying)
32.             {
33.                 // Record the new selected locale so it can persist between runs
34.                 var selectedLocale = settings.GetSelectedLocale();
35.                 if (selectedLocale != null)
36.                     PlayerPrefs.SetString(PlayerPreferenceKey, selectedLocale.Identifier.Code);
37.             }
38.         }
39.  
40.         /// <summary>
41.         /// Returns the last locale set or null if no value has been recorded yet.
42.         /// </summary>
43.         /// <param name="availableLocales"></param>
44.         /// <returns></returns>
45.         public Locale GetStartupLocale(ILocalesProvider availableLocales)
46.         {
47.             if (PlayerPrefs.HasKey(PlayerPreferenceKey))
48.             {
49.                 var code = PlayerPrefs.GetString(PlayerPreferenceKey);
50.                 if (!string.IsNullOrEmpty(code))
51.                 {
52.                     return availableLocales.GetLocale(code);
53.                 }
54.             }
55.  
56.             // No locale could be found.
57.             return null;
58.         }
59.     }
60. }

It will update the preference whenever the locale is changed.