The only way to assess the task of understanding a completely alien language as “pretty trivial” is if you don’t really understand anything about neurolinguistics and the basics principles of communication at a cognitive level. Although addressing the field in even summary fashion is beyond the scope of even a long post, it is worth defining some basic terminology and briefly addressing the essential elements.
The study of language as a cognitive function has three essential aspects, which are syntax (the rules that structure the language), phonology (the characteristic sounds or other signals used to transmit information), and semantics (the actual knowledge content that is transferred). Language by itself is not communication, and in fact many animals from hive insects to birds can communicate in surprisingly sophisticated detail even though they do not have a structured language of any kind. What language permits is the communication of abstract concepts by referencing some kind of shared experience or knowledge base. We are able to talk to other people in the same culture because not only do we have agreement on the syntax and phonology but we also understand basic concepts of experience, both external (light, dark, dry, wet, et cetera) and internal (affective responses such as fear, anger, curiosity, lust, et cetera; physiological sensations such as hunger, pain, fatigue; and cognitive structures such as stories or games).
With someone from a different culture who speaks a different language, we have to learn a new syntax and if the new language is separated enough often a different phonology, but there are always commonalities regardless of how different the cultural experience is; we all have affective responses that are hardwired into primitive areas of the brain and a common physiology, and the vast majority of human societies have pretty close analogues in social structures such as marriage and kinship, tribal or group leadership and following, et cetera. In short, we have some references to relate to that are common to the human experience regardless of what your mother tongue is and what society you grew up in, many of which are core to human physiology, and the cognitive and affective structures of the brain.
However, there is no guarantee of any of this with an alien intelligence. Indeed, we should reasonably expect their physiology to be nothing like us or any Earth life given that it will have developed independently on a unique evolution. They may not experience hunger, or experience the kind of emotions we do in response to stimuli, and they almost certainly won’t have any commonalities in language even if they use a spoken language (and there is a wide array of potential alternatives). Even the supposedly fundamental concepts of mathematics as the basis for communication are not assured; our concept of mathematics is intrinsically based upon discrete “counting numbers” and ways of expressing fractions of whole numbers, which is natural for us but an alien intelligence might see everything in terms of statistical distributions or even more exotic ways of quantifying their experience to the point that even mathematics would be mutually incomprehensible.
Our cognition is highly structured around language as we use it; even when we aren’t speaking to other people, we use language to interpret the world and demarcate it internally into useable categories; for a completely alien species to have meaningful communication would require some common basis for interpretation which may not exist. This isn’t to say that they could not listen to or read words, back out a syntax, and create some kind of cause and effect response, but that is about as much actual communication as your training a dog, and indeed, the gulf in intellect and comprehension between an advanced alien intelligence and humanity might be as wide as that between us and dogs. The notion that an alien intelligence will want to or find it useful to communicate with us is a far more common and unlikely trope of science fiction.
Dolphins are probably not trying to communicate with us, but that is exactly the point; despite decades of experience and research, we can’t even figure out what they are saying to each other, even though the complexity of their verbalizations and degree of encephalization suggests that they are capable of complex communication well beyond merely expressing affective states the way most animals do. But our lack of a common experience makes it very difficult to even guess at what any combinations of clicks and whistles might mean precisely because of the complexity that such language might express. You can hypothesize that an alien intelligence would by dint of their advances have an ability to interpret language that is far more advanced than ours, but even if so it doesn’t mean that they would enjoy sufficient commonality of experience to actually be able to comprehend the semantics expressed through language.
Stranger