The barrier with animals is having an actual vocabulary. They can convey their current demeanor - like it’s generally not hard to tell the difference between a dog wanting cuddles vs wanting to rip your face off. Some of them can even get a little more advanced with things like ringing a bell to tell you that want to be let outside, or bringing you an empty food bowl to tell you they’re hungry, but that’s kinda it. They don’t have a specific bark-word for “Food bowl” or w/e for us to be able to translate.
Couple of exceptions though:
Some of the smarter breeds of parrots can develop a limited working vocabulary. You might be interested in Dr. Irene Pepperberg, a researcher who did a lot of work with parrots. It’s pretty well established that you can train a parrot to say “green!” when you point to something green, but Irene wanted to show whether they actually understand that that color IS green, or if they just know they can a treat for saying “green” when their owner points at something that happens to be green. And it’s the former - she was able to show they actually understand the concept of color + words that represent specific colors.
I think similar work has been done with apes by teaching them some sign language.
But even with the couple examples of animals being able to learn an actual vocabulary, you still wouldn’t be able to have any kind of conversation with them outside of those super basic exchange like “What color?” “Green!” “Good boy!” *treat*.
So back to aliens - specifically aliens capable of space travel - it’s a pretty safe assumption that they’d have some kind of language as advanced as our own, if not moreso. The societal development that would need to happen before them achieving space travel would absolutely require language.
Sure, but I do think, there’s quite a number of animals with some form of vocabulary, like songbirds, whales, dolphins. To some degree, we do understand these, but there’s still huge amounts of vocabulary we haven’t deciphered, nor can actually respond to/with.
And even if those are basic exchanges, I feel like we could learn a ton about the different forms of communications and where our limitations are. Like, ants communicate with pheromones and bees with dances. Ultimately, we’d likely still send out EM-waves, simply because they travel quite well through space, but maybe a bee/alien would think our attempt at an obvious encoding is just background noise.