I think this is a more difficult question than you might first imagine.
There are two sites I think it's worth looking at for comparisons - English Language Learners and Travel. Both of these sites have region-specific tags. ELL only uses these tags when the person asking the question really needs an answer that relates to a specific region. Most of the English language is agnostic to who speaks it, though some terminology and vocabulary differs. Travel is pretty much the opposite. Most questions on Travel have a location tag, either a city, country, or occasionally continent.
So, the question is, which end of the spectrum do we want to be on. Do we feel that Interpersonal Skills are more broadly the same across cultures or do we assume that they're so vastly different that we practically require every question to have a locality tag... personally, I think we are somewhere in the middle.
We rarely really know if something is vastly different between cultures because we don't often realize that these issues are culturally relevant. I remember learning in my Japanese language classes that they never really had an equivalent of saying "bless you" when someone sneezes in Japan - it would largely go ignored - until more recently.
I think that we need to consider how important cultural relevance is to a question. So, if you're from one culture and trying to understand how to do something when plopped a different one, either due to travel or work, or interacting with someone from another culture visiting yours - if you want answers to be very specific to that culture, you should emphasize it by marking the question with a tag.
Right now, though, this is all so new. We need to give the time for the site to develop, outside of private beta and become an ecosystem. If questions start getting too complicated with too many answers, we can consider requiring localization for tags but, until then, I think we should only request tags in situations where the users feel that it would help limit the scope of the question because cultures differ widely or because the user wants answers that relate only to a specific culture.
In the case of questions where the OP hasn't requested a specific culture's response, it may be valuable to have answer-ers note (if writing from personal experience) what culture they belong to so that others may be able to write differing thoughts... I'd bet that even within on country there may be different opinions on the "correct" way to interact in a social situation.
That being said, there should be no "default" assumption. Stack Exchange may be US based and questions must be posed in English but no site that I'm aware of has a "default US" for anything. Cooking doesn't require questions to be in imperial/Fahrenheit, Movies & TV avoids terms like "foreign" as non-descriptive because everyone's foreign to someone else.
I think that a lot of the questions being asked right now are hypothetical rather than an issue someone actually has. I won't call out specific questions but this is a feeling I've been getting. I don't know if this is good or bad... if someone has a similar question, perhaps they now have a good answer to it but if the hypothetical didn't really address the issues they have, now they have to be extra careful in crafting a new question to prevent it being closed as a duplicate. I think I'm more on the "bad" side of the fence. We should let questions come organically, not force them.
Regardless, I don't think we'll "run out" of questions. If we were to, I think we could all happily hang up our hats and congratulate ourselves on solving interpersonal problems forever... sounds like a good thing to me!
united-states, unless the OP stated otherwise. If people think the region in the question is important, ask for clarification in comment, then add the appropriate tag.