Newtonsoft.Json is used by Localify for reliable JSON serialization.

Step-by-step (via Unity Package Manager):

1. Open Unity → Window → Package Manager.

2. Click the + icon in the top left.

3. Select Add package by name...

4. In the Name field, enter:

   com.unity.nuget.newtonsoft-json
   

5. Press Add.

✅ After installation, you can use using Newtonsoft.Json; in your scripts.