This very problem was faced by those who gathered together to work on the UNESCO Charter. A world containing the U.S., the Soviet Union, Britain, China, France, and others is going to have plenty of disagreements. Trying to find a theoretical consensus on matters like human rights will be impossible. It was soon recognized, however, that even whether there is theoretical divergence, practical convergence is possible. As Maritain put it:
Owing to the historical development of mankind and to ever-widening crises in the modern world, and to the advance, however precarious, or moral conscience and of reflection, men have to-day become conscious, more fully than ever before, though still to a very imperfect extent, of a number of practical truths regarding their life in common upon which they can agree, but which derive from the thought of each of them.
That is, while our theoretical views are different, people with different theoretical views can nonetheless have common experiences. They won't understand these common experiences in quite the same way, of course; but they can come to analogous understandings. And for practical purposes 'analogous' is often close enough to 'same' to be counted as such. Thus, even though you and I may have different reasons for saying that people should be protected from arbitrary search and seizure, and therefore not have exactly the same idea of 'protection' or 'arbitrary' or 'search and seizure' or 'should' or even 'people', our broadly common experiences of home, privacy, etc., allow for overlap in practical application. We can both take "People should be protected from arbitrary search and seizure" as a regulative principle, a standing reference point to which we can both appeal. Particular differences in how this is understood can be argued out -- again, to some solution adequate for practical purposes -- on the fly as they happen to become relevant.
Thus Maritain proposed that each group involved in the moral charter should formulate their own Declaration of Rights, laying out the practical conclusions they have arrived at from their particular ideological standpoint. These Declarations could then be compared, the formulae recast, and by conversation and compromise combined into one Declaration, representing a "convergence of practical views". This task, though formidable, is far easier than what would be required to build a convergence of rational justifications for these views. That would require an immense cooperative philosophical labor, one require superior intuitions, new modes of systematization, radical criticism of vast oceans of error (both minor and major), and would still leave us with a result that would need to be put into practice. It would require an immense degree of foresight and wisdom to yield such a universal system, and yet more to make it practical. As a matter of ordinary human nature we can often do better by solving our moral problems ambulando.
Of course, Maritain has his own perspective and operates within his own tradition. So he also gives a set of philosophical reasons for believing this is the best approach, and since I am in broad agreement with him, I'll briefly summarize his points here. The best rational justification or theory of human rights, and certainly the theory that Maritain and I share, is that of natural law. This natural law is universal as far as its general principles go; but it requires a supplementation that takes into account the specific circumstances found at a given time and place. This supplementation consists of custom and positive law which, however, supplement it imperfectly. As time goes on, however, and custom and positive law are rigorously held up to standards of practical reason, they become refined, more detailed, and also more consistent as measured according to the general principles of natural law, which are related to them as ends to means. However, because of the contingency of positive law, it is simply impossible for there ever to be a definitive, final, exhaustive declaration of human rights, because as circumstances change any positive law will be forced, if it is to remain in conformity with natural law, to make relevant adjustments. Thus, while prior written declarations of rights are important, it is valuable for human beings to renew their declarations from age to age. Likewise, it is valuable for human beings in different cultural and political circumstances to cultivate their own declarations. (The divergence that necessarily results from these contingencies is counterbalanced by the convergence that arises from broadly common experiences: experiences like parenting, home, family, work, hunger and thirst, and dying.)
A further corollary of this view is that declarations of rights, not only heavily dependent on contingent circumstances, will tend to group together "rights of varying degreesl" That is, it may have formulations that are among the best approximations of requirements of natural law across society that are available at that time and place (like basic rights of conscience), formulations that derive indirectly from natural law by way of common conventions such as those established in international law or widespread custom (like general rights of property and exchange), formulations that do not derive from natural law but protect from undue influence mechanisms that have shown themselves to be reasonably effective in facilitating common good in general and the preservation of individual rights in particular (like freedom of the press), and so forth. One of the weaknesses of most forms of modern liberalism is that they are incapable of recognizing this distinction among different rights, treating them all as on the level, and are therefore committed to impossibilities: either they are forced to contradict themselves by treating rights as both unlimited and limited, or they are forced into the hypocrisy of treating them in one case as one and in another case as the other as practically convenient. They are, moreover, forced either to treat contingent institutions and mechanisms as if they were general and absolute moral necessities, or to treat the moral necessities as if they were contingent institutions and mechanisms, or, again, waver back and forth between them as they find convenient under whatever circumstances happen to prevail at the time. In fact, we should expect natural divergences in the views people have of these things, arising from different circumstances and histories; standards can be set up that cover everyone not by setting down univocal principles that everyone recognizes as basic principles, but by recognizing in the particular principles each group develops through reasoning on its own particular experiences the analogies that grow up due to analogies among these different particular experiences, analogies that allow us to formulate rules or guidelines that are shared, always or for the most part, in actual practice.
And as with declarations of rights, so with declarations of responsibilities or obligations. And thus we find room for both moral disagreement and moral progress on at least some issues. There is, of course, more to be said on this and other points.
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You can read Maritain's reasoning in his own words (PDF) thanks to UNESCO.
