One of the things that has always frustrated me about Sci Fi is how you go to Planet A and the whole planet speaks the same language and has the same culture and one centralized government. Then you go to Planet B and the whole planet speak another same language and has one culture and and one centralized government.
Today though, my roommate and I were talking about the evolution of the English language and how it grows by absorbing from every other language and how English speaking countries will develop completely different slang from each other. If all these countries where English was the primary language were more isolated from each other, eventually they would evolve into different languages. However, due to modern and ever expanding communication technology, we can all understand each other’s slang (even if as an American I don’t call a grill a barbie I know what New Zealanders and Aussies mean.) Therefore the ability for the language of multiple countries to evolve TOGETHER despite being on opposite sides of the world (for some of us) is unprecedented in our history.
So back to the Sci Fi aspect: If your planet is at the technological point where you have casual space travel, your technology would have had to evolve to a point where global communications were instantaneous and flawless. Where cultural sharing and learning was the norm. As language evolves, so does the culture that speaks it. If multiple countries and cultures were all contributing to a shared language, it is likely that would morph into a shared culture for ease of cultural linguist context (Linguistic determinism. Part of the Sapir-Worf hypothesis). One language and one culture could then easily share one centralized government, especially when if you have the technology to casually travel through space you can OBVIOUSLY do so on your own planet and therefore there is no part of your world that is too far away to govern.
Therefore what I had always felt to be lazy writing on the part of the author or script writer could actually be construed as them thinking so far into a future where, due to highly advanced communications and travel technology, each planet capable of warp speed space travel would have had centuries to evolve into one culture that speaks one language. Which makes Terrans who still have hundreds of different languages and cultures and countries the backwards hicks of space travel who really only got here through bull headed determination and lots of explosions.