Localization: Duplicate Locale Identifiers are not detected when the identifier is a single character

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
4. Click "Add Custom Locale", enter a name and the identifier x, and click "Create Locale" to save the asset
5. Click "Add Custom Locale" again and enter the identifier x a second time
6. Check whether a warning appears and whether the "Create Locale" button is disabled

Actual results: No warning appears and "Create Locale" stays enabled, so a second Locale with the same one-character identifier can be created. The project ends up with two locales sharing the identifier x

Expected results: A duplicate identifier is detected and reported regardless of its length, and "Create" is disabled - the same behaviour as for a two-character identifier

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

Tested on (OS): MacBook Pro - Tahoe 26.6.2

Notes:

• A two-character duplicate is caught correctly, so the check exists but does not apply to single-character identifiers.