Obviously it depends on the use case and audience, but language and location should not be conflated by default. Location allows customization of currency, measurement units, legal policies, shipping information, tariffs, dates/times, etc independently of the copy language. There are plenty of countries with more than one major language (there are almost 40 million Spanish-speaking citizens of the US and India has 23 official languages), so websites that guess language based on country alienate a lot of users.
Your browser already tells every website what language(s) you speak in the Accept-Language header, so it's not like that information isn't available.
My (very subjective) guess, is that many of those 40 millions Spanish speakers in the US speak English as well, and the percentage of those who don't and would use an e-commerce site is low.
Still, I would imagine major e-commerce websites to add support for a langugage in the website, if the potential userbase is big enough (for example, the Apple Store does).
My knowledge of India is very limited , but it always seemed to me that the unifying language really is English (again, from a very cursory glance, Amazon, Apple and HP have Indian e-commerce stores in English).
My point is that decoupling language and location as a general feature does not make much economic sense in the vast majority of cases.
This is especially true for major e-commerce sites: think about Amazon, and the number of items being sold (millions ?). Many of those are sold only in a particular store, or have variations between a store and another: how much would it cost to translate all the articles for all the stores in x languages ?
Your browser already tells every website what language(s) you speak in the Accept-Language header, so it's not like that information isn't available.