Decide if this power has its own language, which will often be named after it; sometimes the power’s name changes, but the language’s name doesn’t, a minor detail that adds depth if we have occasion to mention it. Even if many cannot read and write it, a written language will almost certainly exist. Higher forms of governments arguably require it.
Secondary languages can also be widespread. These may originate from other species. The language of neighboring powers will be spoken by some people but isn’t something we need to consider unless these languages are widely spoken. The presence of other languages could bother those in power to the point of suppressing them, as an authoritative state might. Writings in other languages might also be banned. In extreme regimes, reading and writing might be withheld from the population to keep them ignorant. We should decide whether areas bordering other lands have secondary languages or not.
Humanoid species are often widespread enough that territory isn’t the deciding factor in whether others know the language. Rather, personal experience with that species will cause most people to pick up some words. In such a case, understanding will be limited instead of deep and broad; this is known as a working knowledge of language—enough to get by on the streets but not hold deep conversations. In more educated worlds, a language could be taught in school much as it is on Earth. This might be truer in SF, where education is generally assumed to be far greater than in fantasy settings, but we can challenge that assumption.
Fantasy often includes the concept of a common tongue, one that most humanoid species can speak well. This concept is useful on film, reducing the need for subtitles at the least, but it also frees authors from inventing languages. Despite the nomenclature “common tongue,” not everyone will truly be fluent, as barbaric species are often depicted as being barely coherent. Anatomy sometimes causes this. So can the common tongue diverging too strongly from their natural language so that they sound guttural in speaking it. This common tongue will be the language of somewhere. When choosing a name for it, we might first want to consider what empire of the past conquered so much of the world that its language became the common tongue.
In SF, language is rendered moot by universal translators. Characters are unlikely to learn other languages unless for curiosity’s sake or because the equivalent of Starfleet Academy from Star Trek demanded it of officers. In the event of a universal translator failure, our characters may be unable to communicate with alien species at all without said training. It seems reasonable that on a large ship, at least one person would be fluent without the devices, not by chance, but due to protocol. A ship’s A.I. would serve the purpose, but that can be comprised. The machine translators are typically not present onscreen but are portrayed as largely infallible when that’s unlikely to be the case. Subtle nuances are lost on even people, let alone machines. Relying on just the words and not the tone or body language of the speaker is also problematic.