But there is part of me that asks - should I be more concerned about a worker in Detroit than a worker in Sao Paulo?  Why is the former more important to me than the latter?

Morally there may be no difference. 

Practically--well, we have had this concept of citizenship where we are all members of an aggregation of people that form a political jurisdiction where we have rights--and, ostensibly, responsibilities that are more defined than they are for humans writ large outside of our country/state/locality.  In theory what you and I share with the worker in Detroit is a government that we can indirectly use to solve or at least seek to ameliorate problems. 

The citizenship concept seems to mean increasingly little in practice in the United States, which I see increasingly as a kind of grand economic, religious and cultural bazaar where people choose what they want and in most cases go on autopilot when it comes to giving much thought to others or to what would be necessary communally to sustain, let alone, improve, collective arrangements over time (where there is even any awareness that the collective impacts the individual and must be nurtured and protected, a sense the Randians and others with that mindset have done an enormous amount to erode and destroy in this country).  I see this country as one which occasionally rallies together for a short time in the immediate wake of some disaster or attack, post 9/11 or Katrina, as examples.  But otherwise is highly fragmented and disunited.  On the surface that can work for many when the economic system is working tolerably well and where there is perception of enough for all, meaning primarily living-wage jobs. 

Even when it comes to the traditional scoundrel politicians' tactic of rallying people to the flag for wars when domestic or personal trends prove too unwelcome or embarrassing, there is a growing danger of crying wolf in the US.  We get involved in more and more overseas conflicts which the ordinary citizen questions the relevance and necessity of, but can shrug his shoulders because it's mostly other peoples' kids, volunteers, who do the fighting and dying.  For the most part the ordinary citizen has little or no skin in any of these conflicts.  Meanwhile the pillars and foundations necessary for the maintenance, let alone health, of the collective, erode.

I'm currently reading a book on collected writings of Eleanor Roosevelt. Some of these naturally contain some of her thinking re the Universal Declaration of Human Rights which she did so much to bring about.  She writes at one point about how in the US we see the "natural" social unit as the family.  She writes that in the hunting and gathering phase of human living the clan was seen as an extended version of the immediate family. 

Then over the millennia we nominally aggregated into larger units of association, leading eventually to nation-states and countries and even some regional governmental units as in the case of the EU.  And the thinking of idealists such as ER was that if we could learn how to care for one another in increasingly larger aggregations we could move towards including all of humanity as that aggregation, over time.  WIth real, meaningful human rights.  That was the vision anyway.

About the prospects for that much could be said.  I think the larger environmental and economic context matters a good deal in what the dominant mindset is.  If the economic system in conjunction with the environment and natural resources available is seen as offering potential abundance that is one situation where, potentially, more people can see grounds for hope and optimism about the future. 

Where that is not seen as the case, on account of failed or disfunctional economic and/or political systems, and growing environmental/resource constraints, it fosters more of a dog-eat-dog outlook less conductive to the kind of fellow feeling probably necessary to some degree for effective community.  Plus I think it is true that where there is no vision, the people perish, and there is no vision being offered by any political or economic or cultural leader today with stature that provides anything like a widespread sense of hope and positive direction for the future.

I think in the past there was no prospect of humankind doing ourselves in entirely or anything close to that on account of environmental destruction.  So that means we need to view the present and future with a very different lens and mindset than has been the case since Hiroshima and Nagasaki.   

In the race between humankind's moral evolution versus the technical and environmental and political challenges we face, we are in great danger of failing to rise to these multiple challenges, with the consequences of failing to do so far vaster than in earlier historical times.

The European social solidarity mindset is impressive to me, warts and all, and even taking into account the relative advantages of smaller national scale and greater homogeneity which tests humankind's ability to accept the other far less than is the case in, say, the US (although with the influx of Muslim immigrants we see the strains that is imposing there).  But it, too, will eventually be vanquished on the national and regional scale in the absence of a reversal of the current trend of humanity polically/socially losing the race against the environmental clock.

I think your question is a very good one.  I think that the times we live in now require us to look at it in a different way, because we face unprecedented challenges which if not met will, I think, tend to erode the historical distinctions between societies which have figured things out and sustained themselves relatively well, versus those which didn't.

should I be more concerned about a worker in Detroit than a worker in Sao Paulo?  Why is the former more important to me than the latter?

A society survives and prospers thanks to people not asking those questions, thanks to a sense of empathy, common interest and mutual duty. You lose those, you lose your society. If one starts casting about for a theoretical - prudential, deontological - reason to feel this way about a worker in Detroit, its already pretty much a lost cause, and we're reduced to doing a post-mortem analysis of the fall of a society, a zombie civilization.