You mightn't be able to understand them
Yes, the people in Italy speak different languages than the people in America.
So you can speak to people who don't speak your language.
They speak a different language, Slovak.
They speak a different language, Slovak.
So you can speak to people who don't speak your language.
If you can only speak one language that's fine because it suits the environment that you live in but if you speak more than one language that's even better because you can speak to foreign people and will be able to speak a particular language when you go to a different country.
The French speak French because it is their national language, historically developed from Latin and influenced by Celtic and Germanic languages. French became standardized as the official language in France due to cultural, political, and historical factors.
Not necessarily. A language family can have dozens of different languages in it.
In Communist (mainland) China. Mandarin is the official language and spoken by the majority of people but differing regions speak different languages.
Serbian people speak the Serbian language.
You should always speak the common language of the place you work. If your native language is different than the common language, it is counterproductive and rude to speak it at work.
Many people in Melanesia speak a pidgin language due to historical interactions between different language groups, such as colonization and trade. Pidgin languages often develop as a simplified form of communication for people with different native languages to interact.