Safari showing website in different language than what's set in Preferences
Regional Settings in Preferences sets your "localization" preferences.  In other words, it sets the language, the decimal notation, the date format, etc. for your local computer.

It does not force remote computers to conform to the standard you selected.

Whether or not a website displays in your preferred language has to do with the website/web developer and whether or not they chose to implement a translation mechanism or alternate websites based on country of the visitor (this is usually done by a combination of location and browser agent string).

If the website you visit doesn't provide either an English version or an automatic translation feature, there are online translators available.  I prefer to use Bing Translator