When adding a new locale to a B2C Commerce site, it is important to first add an alias for the new locale. This step is critical as it helps in defining a unique identifier that can be used in URLs and for other localization purposes. After adding the alias, the developer must then create and configure the locale under the Global Preferences section of the Business Manager. This configuration involves setting specific parameters such as language, currency, and country code, which are essential for the correct functioning of the locale on the storefront. Activating the locale in the site preferences is also crucial to ensure that the new locale is available for use on the site.