Assuming that some form of faster than light travel, or some means of side-stepping the immense distances between star systems, is possible, we will, if we haven’t already, encounter intelligent civilized alien species from other star systems. The alien species on Star Trek are almost always humanoid, with roughly similar technology. Their guns might shoot green light beams while ours choose red, but otherwise they’re pretty similar. Almost all speak English and the few that don’t can be spoken to with a universal language translator.
I expect the reality will be much different. Given the universe has existed at least 13.8 billion years, and if you don’t believe the big bang model, possibly much longer, the odds are good that the vast majority of species that we encounter will be considerably more advanced than us, they won’t speak English, there will be significant differences in their body plans, and not all will even be humanoid. There won’t be any such device as a universal language translator that can instantly translate an unknown language into perfect English and English back into that unknown language.
However, I don’t think it will necessarily be as dark as some people anticipate. Many people consider the Earth’s history and what has happened on Earth when civilizations of unequal degrees of technological advancement meet. It has universally not been a positive experience for the less advanced civilization. But there are some major differences between these encounters and extraterrestrial civilization encounters that may occur.
The population on Earth has for all of recorded history been pretty close to the maximum that the Earth could support given our technological status at the time. Improvements in technology such as irrigation, selective breeding, fertilizers, mechanical planting and harvesting, have allowed us to considerably increase the productivity of the land and thus to support larger populations, and the populations have always grown to what the new technological limits support. So man has constantly always been looking for more resources and that makes it bad for the lower technology civilization when it encounters a higher technology civilization because in a competition for resources the lower technology civilization always loses.
If extraterrestrial civilizations can get here, one of two conditions is true. The travel from a distant star is expensive, in which case coming here for resources will be too expensive to make it feasible on a large scale and thus they will not be a significant threat to our civilization. The other possibility is that interstellar travel for them is cheap, in which case they will have access to such a vast amount of resources from asteroids and uninhabited planets that they will have no incentive for taking our resources.
Any alien civilization we encounter will most likely have survived element 92, which means the type of alien civilization portrayed in the film Independence Day or in the movie Alien most likely would have self-destructed before they developed interstellar space travel capability.
That isn’t to say there are no threats from encounters with alien civilizations. There are many possibilities that could be threatening in ways not intended. For example, the possibility that an alien species visiting our planet might unintentionally introduce invasive species. Just as our cargo ships often unintentionally bring invasive species that either cling to the ship or are taken up in ballast water, or board the ship in one port and leave in another, so might alien species from other star systems unintentionally bring something here that thrives in our environment at the expensive of native species. I’ve wondered if the chupacabra wasn’t some alien kids pets that got loose during an excursion to Earth, a pair that bred and propagated.
Another way interaction with an extraterrestrial alien race could be negative, and I believe this in fact is happening, is that if they provide technology to only a select few, and they give just enough technology for those select few to use it to take resources from other humans and not enough to go mine asteroids, or to reach other habitable environments, then those few will weaponize the technology and use it to control fellow Earthlings and steal their resources.
I hope that if an alien civilization opts to interact with us and share technology, they do so in a way that makes it broadly available to us so the temptation to weaponize it and use it on each other is minimized. Unfortunately, there seems to be much evidence that exactly the opposite has happened, it’s been given government military people who only see technology as a more efficient method of killing people.
At any rate; language is going to have to be learned, just as it is when terrestrial civilizations meet. Odds are good that it will be more difficult because the alien languages won’t have common roots with Earthly languages. The speech apparatus, if it exists, might be entirely different making entirely different vocal sounds that we can’t produce or possibly even hear. Written languages might be similar or entirely different. The human brain has a natural syntax that at least some human languages employee. Alien civilizations might have a much different natural syntax.
If alien biology developed independently, then the chances of aliens and humans mating and creating hybrids, like Warf and his human female companion which mated to create a half-Klingon half-human hybrid, not likely. Not only would DNA not likely be compatible but cellular machinery which interprets the DNA would likely be incompatible.
Another thing I think is that we’ll not see them here in great numbers simply because the universe is so big, there are many places to go.