One of the great cultural complexities of language use is the knowledge of when you can use a term. It is not only a matter of meaning and register (formal or familiar) but also of status. For example, in English, every idea has at least two different words to express it, if not more. Sometimes, the choice of word depends on your relation to the person to whom the idea is addressed.
It may be depend on your gender. For example, if a woman tries on a dress that may have fit her when she got married but that was many years ago, a man asked his opinion of the dress in front of the salesperson and feeling obliged to tell the truth would have a hard time phrasing his answer in a respectable manner. He might say “It’s a bit small on the top.” A woman, especially a friend, can get away with being direct: “Your tits are hanging out.” Brave is the man that tries that line. While men can use the word “tits” behind a woman’s back or in a certain context at home, woe is he that uses it in front of her in public.
Likewise, ethnic labels are sometimes reserved to the member of that ethnic group only. The classic is that outdated racist term “nigger.” The fastest way for a white person to get abused, verbally and/or physically, is to use that term directly to an African-American, correctly so as the expression is insulting. However and most peculiarly, black comedians can and often use the word when referring to their “brothers”. This reflects street use by some African Americans among themselves, when it is not intended as an insult but instead of a social comment. As Richard Pryor once said, there are no niggers in Africa (but there are in Detroit).
Finally, there are some terms that we use in private, only to be spoken directly to ourselves when nobody else is around. Countless people get up the morning, look at the mirror and say out loud “you are fat, ugly, dumb and lazy,” or at least some of those terms. If anybody else made that comment to us, we would find it quite rude. Yet, rules of etiquette don’t apply to conversations with private conversations. Terms such as portly, big boned, learning challenged, and a bit slow, to name a few have no place in our private world nor should they. As Detective Friday used to say, say the truth, the whole truth and nothing but the truth.
So, knowing the right word to use is often much than a matter of linguistic knowledge. Cultural knowledge is often also required.