Localization: "Add Custom Locale" warns that an identifier is already used by a locale that does not exist in the project

Steps to reproduce:

1. Create a new project
2. Install the Localization package (com.unity.localization)
3. Open Edit > Project Settings > Localization and click "Create" to make the settings asset. Do not add any locales - "Available Locales" reads "List is Empty"
4. Click "Add Custom Locale"
5. Type "fr" in the Locale Identifier field
6. Read the warning message and review the state of the "Create Locale" button

Actual results: "Create Locale" is disabled and the warning reads "The Locale Identifier is already being used by French", although no French locale exists in the project. The statement is untrue as written, and nothing indicates that the intended route for a standard culture is "Add Locale" rather than "Add Custom Locale"

Expected results: The message distinguishes the two cases — an identifier that matches a standard culture, with a pointer to use "Add Locale" instead, and an identifier already present in this project

Reproducible with versions: 1.5.12 (6000.5.7f1, 6000.6.0f1, 6000.7.0a6)

Tested on (OS): MacBook Pro - Tahoe 26.6.2