Doc: I really appreciate this Professor Mukh.
Mukh: Please, no need for formality. I think it's sweet you want to learn Lurran for Riasi. Most humans think if you speak Terran loud and slowly aliens will just understand it.
Doc: English. It's called English. Where did you learn to speak it?
Mukh: Correspondence course.
Doc: No offense but it couldn't be very good.
Mukh: If it were any good do you think I would have picked Tiglath Alfstan as my first and middle names? I got better.
Doc: You did.
Mukh: Now, do you speak any Lurran?
Doc: Sure. I can say, "Good job." Hreeeeshereeet!
Mukh: It means, "My fur rubs the other way ... asshole."
Doc: ... That could explain a lot actually.
Mukh: It is a very concise language.
While aliens al seem to speak English on television that may not be the case. For one thing they may opt for Chinese or Spanish as more people speak those languages on modern Earth. Aliens might be well advised to learn English if they plan on obeying locals laws for landings and such as it is the international language of air traffic control.
They might also learn the language of those people with a space presence. That would have been English and Russian a few decades ago. With al the communications media routed through satellites these days that isn't much help. Everyone has some media being broadcast via satellites. For that matter they might communicate in the manner of the Curiosity rover on Mars (or Odyssey) since these robots have gone the farthest of any space travelers.
Communicating with aliens is bound to be hard though. I'm not talking about the scientists and ambassadors with the batteries of computers creating a common mathematical language. I'm talking trader Fred trying to cut a deal for those pretty rocks.
Humans have a bias towards verbal communications. We run into problems even trying to talk to our best friends dogs. Canine communication is 90% non-verbal: movement, position, ear and tail motion. In humans it is 90% verbal and we often resort to hollering the same instructions over and over when a simple gesture might suffice. In spite of this dogs manage to learn dozens of human words. I can only identify one bark of many my dog uses ("WTF are you?) for summoning his protege.
Alien vocal apparatus may be capable of making sounds we simply can't or discriminating between sounds we can't. This is without even getting into beings that have different hearing ranges than us. You might be agreeing with that Antarean potentate but the subsonics in your voice indicate satirical intent. Beore you know it the giant robot is unleashed.
Again that is just hearing. An alien might add pheromones emission or color changes to its communication methods for all kinds of embedded meanings. Sure the comic relief aliens never gets the hang of or uses tenses because he's changing his facial coloration to cinvey them. You can't see into UV? Not his problem.
Note that aliens with tails or emotive ears are cute but probably going to use those in some kind of communication and be a problem to talk to. Our languages might sound clipped or redundant to them. An ear flip or tail flick could be worth a thousand words.
Note that I'm still talking about relatively 'human' aliens. A lot of people seem to think aliens are all going to be incomprehensible and unable to communicate with us. We don't really know and that's no fun for an RPG. One or two incomprehensible beings in a campaign are plenty.
Some forms of communications may not even involve sight or hearing. What about beings that sense and generate electrical or magnetic fields and use them to talk? If they switch from AC to DC current run. Beings with sonar sense might move the ricks the swallow in their gulets to communicate.
Humans and aliens might have better luck communicating with pictures (assuming our vision is similar. If you need to record something simply writing is one easy way to do it across many levels of technology. No matter what exotic mens your have to talk writing it down boils it all down to the sense of sight even if the human needs a black light to see what he's writing in day glow.
A race with using radio communications might have trouble adding new signals to represent pheromones or color changes and resort to something like a sound track they trigger manually this could be adoapted to the poor bland, scentless humans' needs as well.
Universal translators are very popular in some stories, especially where you have only 40 minutes a week to tell your tale and language lessons will eat up time for commercials. they work using ... SCIENCE! Apparently every just hears the alien speak English after it's turned on. I'd still like to see one of the more continuity minded SF series have an alien survivor or refugee come aboard the crew and spend several shows learning to communicate.
One way of emphasizing you are dealing with alien intelligences is to take the translator out of the background. Put several variants in. Have groups parleying for a few rounds and fiddling woith the settings on their translators to get things right adds color. Why do the translators always have to be tiny ear buds or badges? Why not a set of glasses with heads up displays that provide subtitles. The HUD display would also have the benefit of color coding text to indicate the reliability of the translation or emotional state of the speaker. Slipping a voice (or whatever) stress analyzer in there might occur to certain untrusting souls.
Human built translators might stop at translating vocal language with hilarious results. How important can those ear wiggles be? Aliens in a galactic community might also adopt a pidgin language free of the more subtle meanings and making them sound a little simple minded. For a hilarious example of this check out Drakk in the Irregular Webcomic site. He's a reptile man who speaks common sounding like a moron. Drakk is quite eloquent in his native tongue (which doesn't translate exactly into common due to different sound ranges (he hears many hypersonic sounds.) They might also be using low price translators or just lag behind other races in translator doubletalk. If aliens have already built tiny and efficient translators this could lead to the humans believing they all speak English and explain a lot of episodes of Lost In Space.
Perhaps the phobia against psionics in so many settings is engineered by the Transalienator Corporation, out to protect its galactic monopoly in translator systems and software. Why should the pharmaceutical, munition and mining corporations get all the good conspiracies?
Nok: A bad fur rub. Terrible.
Tivk: I will have to take my leave of you gentlebeings that I may laugh privately. Excuse me.
Nok: Wrong way fur rubbing is a serious ... you body-bald snozzwanger.
Tivk: <Snerrrrrrk> Excuse me!
Mukh: It's too bad human translators suck so bad. Ours are much better.
Nok: Shhhh. Keep them humble. Let them think we all speak English.
Mukh: Hreeeeshereeet! ... I actually cleaned it up a bit.