They don’t call it localization for nothing! A Locale is the specific target market and language variation translators specialize in, if they work with a language that doesn’t exactly fit one flag and / or geographical location. For example, if you need a translation into Portuguese, you can choose one of the countries or territories where Portuguese is spoken as the Locale for your project. That also means you wouldn’t exactly choose Germany as your Portuguese Locale.

Locales matter because cultures differ, so if you’re localizing your content for, e.g., the Mexican market, you’ll want it to be done in the language your customers speak – and that’s not European Spanish.