Wait...what is US Spanish as apposed to regular Spanish?
Former technical translator here.
In website language selectors, often what you’re seeing is actually a combined language and country selector, such that you are simultaneously selecting the user interface language
and the localized content. For example, you might show only the products sold in that country, and change the contact information in the footers.
I currently live in Switzerland. When choosing the local site, many websites offer (among others):
France
Germany
Switzerland - French
Switzerland - German
That’s how I would word it. But sometimes they say something like:
French
German
Swiss French
Swiss German
Or they just use little flags. (Which can have accessibility issues for users of assistive technology.)
As for why Spanish is different: there are many dialects of Spanish. But as far as written Spanish is concerned, one can consider one set of formal grammar. In many cases though, you must also document colloquial usage, which is why you see that spell check dictionaries come in a few variants. Speech recognition and text-to-speech of course must be very dialect specific in order to have a high success rate.