Culture settings include language, date formats, number formats, and currency. For example, UK and US handle date, number, and currency differently than the each other, even though we both speak English. Changing the culture setting for a site will change the data, number, and currency, and language for notifications. The culture setting does not change any text entered by the Pressero admin when the site was created. That should be changed by the customers browser settings. And, there's the secondary option to force the language to match the culture, which is otherwise driven from the browser in the site settings area of admin.