Is there any StackExchange/Area51 site where it would be appropriate to ask questions regarding computer programmer salaries and how to negotiate them?
Is there something about your question that's specific to computer programming? Or could you replace the term "computer programmer" in your question with "chef" or "accountant" or some other job title? If it's specific, Programmers SE; otherwise, no.
For a how-to on negotiating programmer's salaries, I would think programmers would be appropriate. A lot of the great questions and answers there are how-tos and this can be asked in a non-localized manner. Just be smart about how you ask your question though. If you mention something along the lines of "How much money should I be making?", that sort would be closed (anywhere).
Before doing so however, you might want to ask on their meta to see if that would be appropriate just to be doubly sure.
There are really two separate topics:
Neither one of these topics are prima facie on-topic for Programmers.SE.
Salary negotiation isn't a programmer-centric topic: nothing about negotiating a salary requires programmer expertise. There might be questions related to negotiating a salary that might have some special programmer-only insight, but it's highly unlikely that a question merely about negotiating a salary requires that.
Similarly, scoping a question with "(as a programmer)" doesn't automatically make it a question that demands uniquely programmer-centric expertise.
And our FAQ even explicitly classifies salary questions as off-topic:
So please don't ask them there. We can't help you.
A larger point to make is that some topics don't fit the Stack Exchange model well, or don't have a home yet. Salaries and salary negotiation are two of those topics.