I've developed my answer this question on basis of my response to a related but different question here: Generalize Article I of The Universal Declaration of Human Rights
I believe that:
All life comes into its own as free and equal in dignity, rights and
consideration. Such life as is endowed with reason and conscience also
must observe this truth in transactions with any other form of life.
As an individual that also places value on non-human life, the restriction to human beings seems relatively arbitrary. Moreover, the usage of brotherhood comes off as a bit chauvinistic. All equal in dignity, we're just also going to ignore the existence of women and non-binary transgender identifying individuals in the next sentence. Starting with these two targets, I worked to modify the statement.
My personal adaptation would shift from usage of human beings to life. Fortunately, for extension purposes, life isn't defined exceptionally well right now, but definitely includes humans. Unfortunately, that opens up what should be a relatively straightforward statement to a certain degree of discussion. Oh well. That's why there's a whole stack exchange to discuss this issue.
Likewise, I append consideration to dignity and rights. This has the purpose, in human systems, of also mandating treatment in excess of the minimum required, and in non-human systems, of acting towards other life in accordance with its specific conditions and needs. That is to say, when I take consideration of other humans, I give of my time and resources to others. When I take consideration of other life, I choose vegan dietary options such as do not harm others and respect eco-systems even at non-zero financial cost.
Moreover, I can conceive of both Homo sapiens that may or may not be captured by the term "human beings" that lack reason or conscience and, for those reasons, should not be held to the same standards as other human beings. Likewise, with the extension beyond human rights, restricting reason and conscience to humans in particular becomes meaningless. For that reason, I developed a special class characterized exclusively by capacity for reason and conscience, and charged it with the implementation of my statement (as no other life would be capable of reading, understanding, and acting on it).
I'm left with something that isn't really a Universal Declaration of Human Rights, but, in my opinion, largely subsumes the Declaration, and also address broader and more difficulty issues with its provision for just treatment of non-human life.
But then they consider personhood to start 6 months after birth and
even have restaurants that serve newborn babies.
This doesn't seem substantially less arbitrary than birth or conception from a purely theological viewpoint. I always thought that life could arguably considered to begin with the termination of breast-feeding or on first procreation just as easily. Six months seems fairly reasonable here.
I would argue against on the basis of potential social value of six-month-olds. My generalized declaration mandates consideration by rational agents, and this would require procreation in order to sustain and reasoning population for purposes of management. Likewise, eating a six-month-old, despite the six-month-olds debatable level of mental development, would likely be an infinite negative on that agents utility curve, and therefore is exceedingly unlikely to be a possibility if optimizing in global utility (which I consider a reasonable implementation of consideration). I believe this forms a reasonable, open, and non-visceral reaction that could be addressed through dialogue and should violate no cultural rites.
Or maybe a civilization where bestiality or necrophilia is common even a traditional practice in their society? Maybe people consider necrophilia to be a way of "honoring the dead".
Bestiality suffers from problems with consent, but given some degree of technology to allow communication with animals really has no direct reason for restriction. In my limited experience with bestiality law, in general seems to be implemented only after someone (of any species) is hurt for irresponsible practice of bestiality, but we don't ban human sexual intercourse for similar reasons, so I believe this is really a question of technology on consent and animal welfare.
Necrophilia only suffers from global utility loss in spread of disease and personal reservations within the community. If their communities had no reservations with necrophilia I believe there would be no reason to ask them to change their practices, and we should certainly not impose any arbitrary restrictions on them. Likewise, we should open our hearts and minds to this custom - it seems it could easily be rude to not allow them to honor our dead for exclusive reason of our own personal discomfort with the idea.
Or what if we run into pedophilic aliens?
Like bestiality, this suffers from problems with consent, but in addition risks mental damage to future rational managing agents. Bans on pedophilia are something that I would be more comfortable enforcing because the degree of social consequence, but, of course, it worked fairly well for the Greeks.
From a strictly game theory perspective, on first contact I would immediately move for comprehensive preemptive strike. Interstellar civilizations would have very little difficulty in causing limitless destruction to each other, and consequently the only reasonable degree of protection against catastrophic miscommunication or other faux pas would be immediate and complete annihilation of any technological proficient agents.
I do this on basis of equal consideration as I consider this a charge to management - rational agents are the protectors of life first, and specific life second. This is the only guarantee for protection of life (removal of mutual destruction from the possibility table) and while technologically and morally costly, has a finite cost compared with a probabilistically infinite cost, which for any non-zero probability must still be avoided by a risk-neutral agent.