https://forum.unity.com/threads/nested-localizedstrings-dont-seem-to-work.1203964/#post-7690903

Using
NewItemUnlocked["my-nested-string"] = new LocalizedString();

will throw an exception if the item is already added.

using UnityEngine;
using UnityEngine.Localization;
using UnityEngine.Localization.Components;

public class TestLocalized : MonoBehaviour
{
public LocalizeStringEvent Localizer;
public LocalizedString NewItemUnlocked;
public LocalizedString ItemName;

void Start()
{
NewItemUnlocked.Add("my-nested-string", ItemName);
NewItemUnlocked["my-nested-string"] = new LocalizedString(); // Will throw
}
}