Since this is a recommendation that applies to most any question about Spanish, unless otherwise specified... I think the best place to include it would be the How To Ask help section, specifically here:
If you ask a vague question, you’ll get a vague answer. But if you give us details and context [like target Spanish dialect, tone, examples of the usage you're looking for], we can provide a useful answer.
Something like the bold part (which I just added) might do it. However I don't think moderators can edit the contents of the How To Ask page :(
Another option would be to edit this part of the Tour page:
The example question could be changed to one that shows regional tags like colombia or regionalismo.
Of course, the option of closing on-topic questions (which you seem to like a lot :D) for minor details like this one, is totally unacceptable to me.
We are here to answer questions. Specifying the target country is a tip for the questioner, in case they want a specific answer; but said info is definitely not a sine qua non for us to contribute an answer (as seen in the very same question you linked).
I don't see a problem in a general translation question getting several regional answers, I think it enriches the questioner's knowledge and shows the stack is alive and kickin'. Who knows: maybe OP is writing a book and their main character was from Spain but, thanks to our answers, OP decided that Mexican Spanish sounds cooler!
So in my opinion, inclusion of any kind of details that are not necessary for a question to be sufficiently understood and answered, should be just a recommendation.