Star Trek style games have a couple questions at their heart; why are humans so prominent in their version of the Star Federation, and how do all these aliens communicate with each other? I tied the two questions together for the answer I use in my Lasers and Feelings game.
The first race that meaningfully interacted with humanity quarantined Earth until they got the results from studying the human race. The results came back that everything from the cultural leaps of humanity to their basic psychic profile was toxic and dangerous to the galaxy.
The Peacekeepers that discovered them did the only merciful thing; they virus-bombed the planet from space. The results were not what they expected. About 30% of the human race survived somehow. Among the survivors, over half had mutated brains from the effects of the virus.
These mutated humans were called explorer class. Their psychic contagion adapted so that they could understand the thought patterns and instinctive communication of a range of aliens. With a little practice, explorer class humans could understand alien speech as though they were reading subtitles, even across electronic media or through barriers. Also, those they interacted with were able to understand them at an instinctive level.
This “transthink” mutation affected about 70% of the aliens the Peacekeepers interacted with. Scientists among the Peacekeepers brought their political clout to bear, and explorer class humans were to be harvested and studied. Humans being what they are, within a few years enough had escaped to make alliances and wage war against the Peacekeepers that tried to destroy them. After a century of war, the Peacekeepers were driven to the edge of extinction, and humanity stood at the center of a new galactic coalition.
All that was over 700 years ago.
Now the Star Federation is a sprawling bureaucracy glued together by explorer class humans who are the beating heart of diplomacy, trade, and warfare across the diverse races of the galaxy.