This question is too broad.
Please write another question which is more specific - which culture do you come from and which culture are you worried that you will offend?
Here is an example of a SPECIFIC question which will be quite easy to answer:
What are some words or phrases you should avoid because they are OK in English but would be rude or vulgar if used in China
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Any phrase with the word Jew in it...
You got Jewed. (cheated)
How much can I Jew you down for this (haggling)
The phrase gypped (jipped)...
I don't want them to jip me. (This refers to Gypsys (Roma) and their stereotypically cheating nature)
Real people use idioms and slang, so writers use them to make their characters more believable.
It doesn't mean anything except the fact that they are speaking their language. All languages have slang and idioms. It means that they are using saying from their dialect.
They are hard to understand, so you don't want to be misunderstood.
Offensive Slang: homosexual or gay. Other: odd, strange, etc.
It is under Literary Devices and Figures of Speech - that is under English Language, which is under Literature and Language.
Yes, every language has slang and idioms.
Because they have different experiences. Slang comes from your culture being different from other cultures.
Wherever people lived -- people have always used idioms and slang.
Nobody. Idioms and slang evolve naturally as people talk.
Idioms originated from various cultures and languages over time. They were developed as a way to convey meaning in a more colorful or metaphorical way. Many idioms have roots in historical events, literature, or common experiences shared by a particular group of people.
Try these links
Not as slang or idioms, no. It just means to strike a ball, either with a bat, a stick, a racquet, or some part of your body.
When the English language developed. People have always used idioms and slang in their speech.
Idioms and slang are informal language, and should probably be avoided in business situations.
feeling under the weather
Real people use idioms and slang, so writers use them to make their characters more believable.
Every language has idioms and slang. People just naturally invent ways to make their language fun and interesting. Idioms also add emphasis to a conversation. One of the reasons that learning a new language is so hard is that you have to learn the slang and idioms of that language.