If you're creating a nation - for fantasy or science fiction - I'll begin by encouraging you to give it a language. But even if you have already, don't stop there. One of the things you find all over the world is that people who speak a particular language have strong attitudes about it, both internally or relative to other languages of their world.
Today my husband and I were discussing France and the French reputation for being prickly toward Americans - something which I have never in my life experienced. Interesting, isn't it? Because I speak French well, I always get lots of credit for it. My theory is that Americans and French are very similar. The people of each of these two countries are very proud of their language, and because it is spoken in many countries of the world, they feel that others coming to visit should have the courtesy to learn some of it. This may or may not jive with the experience of some of you, but nevertheless, it's an example of manners which are closely linked to language pride.
In Japan, they have a different kind of language pride. Both my husband and I have encountered situations where we were told we spoke Japanese "too well." There's a strong cultural view of Japanese as a unique language that can't truly be captured by a foreign speaker.
Speakers of different languages can also have varying attitudes toward the use of dialect by people from different regions - some laugh at them, some think they're precious, and others disparage them. Some countries have a national institution whose job it is to maintain the "standard" language against the intrusion of dialectal usages or foreign borrowings (especially foreign borrowings).
I encountered a funny article recently about German train stations replacing signs written in English with ones written in German. The part that was surprising was that the English they were replacing wasn't the kind Americans would necessarily find easy to understand - it was very idiomatically appropriate to a German context. The article is here.
If your world has nations and languages, then considering language attitude on some level will help it feel a lot more real. Even if you've got one language that is the strongest across a whole world, consider that language use diversifies very quickly. English is very strong as an international language, but there are lots of different kinds of English. What is Standard English? How does it compare to the Queen's English? Is one more often learned, or more highly valued in a particular location? If you meet someone from Hong Kong, their English will probably sound British, but someone from the Philippines will probably sound American. If you want to teach English in Japan, it will be easier to get a job if you sound American or British than if you sound Australian.
War is another context in which language control can play a huge role. Take the example of World War II, when Japan occupied Korea and outlawed the use of Korean in public. Korean didn't disappear, but a whole generation of people learned Japanese as a conqueror's language. Imagine how that influenced attitudes about Korean and Japanese!
Think also of the language Hebrew, which was primarily used as a literary language and was then revived for active use starting in the end of the 19th century (source: Wikipedia entry on History of Hebrew). Now it's the native language of millions in Israel.
I hope all of these real world examples can help you extrapolate for situations in your fantasy and science fictional worlds. Language isn't just a tool for conveying messages, but also for conveying information about culture and identity. It can serve conquerors, or rally the oppressed. It can be a measure of refinement or lack thereof. It can be a symbol of national unity, or a symbol of national diversity, or yet again a symbol of deep national history.
It's something to think about.