Every Catalan (or Galician, Valencian, Basque) speaks Spanish, as do the vast majority of the expats I know (and I'm one myself).
You can still access the specific site of the language you are interested in. Or, in the worst case, use Google translate.
Some of those regions are known for being proud of their language and culture, so in case of similar specs and cost many people will chose a vendor that covers their native (or L2 but local) language instead of one that doesn't. Something to take into account is that e.g. Catalan (or Valencian or Balearic, you name it) represents between ~9M and ~11M native and L2 speakers (depending on sources). What else... Oh yes, the regions where Catalan is official in Spain are among the wealthiest when considering the average income for its residents. That sounds like a good pond to fish for early adopters. Basques with even less population (and less L1 and L2 speakers, even in % than Catalans) are also among the wealthiest. The use case doesn't seem that narrow anymore, doesn't it?
Every single big company in Spain is able to communicate in any official language. Dude, it means business!
Finally, and I'm leaving a lot of stuff behind, people are usually not very thick skinned and calling the support of these languages "very limited use case" could be considered, well... inconsiderate at least.
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.
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 ?
I've been doing eCommerce for a long time now and the number of site that realize this is shockingly limited.
With a few exceptions, the translations you are using for a country website's language are possible to use for any country where a visitor wants to use the language. Exceptions include things like country specific product features and such. Those kinds of things would need a little though.
The data is all there and the tech is all there. It's just a desire or realization that is missing.
But apart from the data and tech you need people implementing it, and I can understand most companies not seeing covering your/our case as worty of the added development/maintenance cost that it would entail.
Other companies are MUCH worse. Apple and Google being good examples. Try either being in the US or having a US credit card and purchasing in the EU. Nightmare!
What if I live in Spain and only speak English. What if I live in a Catalan region and want Catalan? What if I live in GB and want to read it in Spanish as it's my first language?
With the world being as global as it is and people readily moving around, geographic location does not equal language preference.
Ideally the site would have geographic specific sections but allow all languages it has translations for in each section. Bonus points if you default language based on my HTTP headers and allow session based overrides.