tl;dr: Unless you're posting on a language-related site (e.g. French Language) or a site where all questions are expected to be in a different language (e.g. Stack Overflow in Spanish), yes, all posts are expected to be in English. A special case exists for the Russian language; see below.
What is the official policy in simple terms?
The Stack Exchange Trilogy has an official policy on non-English questions (please feel free to read the full blog post for more detail):
It is not, nor has it ever been, our goal to be the one place in the world for all programming information in every possible human language.
- Direct question posters to native language resources.
- It is not the community's goal to teach English.
- The question asker should put some effort into the question.
As long as the question is in salvageable English and makes some modicum of sense, it should be edited and improved like any other post.
Why have an English-only policy?
Languages by total number of speakers
English is as close that we have ever come to a global lingua-franca. It is generally considered the dominant language of science and diplomacy. It is also the most taught second-language by a far, far margin. English is the official language of about 45 nations.
English is everywhere. Some 380 million people speak it as their first language and another 600 million speak it as their second. A billion are learning it, about a third of the world's population is in some sense exposed to it and by 2030, it is predicted almost half of the world will be more or less proficient in it. It is the language of globalization - of international business, politics and diplomacy. It is the language of computers and the Internet. You'll see it on posters in Beijing, you'll hear it in pop songs in Tokyo, you'll read it in official documents in Prague. Deutsche Welle broadcasts in it. Bjork, an Icelander, sings in it. French business schools teach in it. It is the medium of expression in cabinet meetings in Bolivia. English is now the global language. - Oxford Seminars
The number of people who speak English is nowhere near as important as how many people can write it. More specifically, the number of people who use their written language on the Internet can be the single most telling piece of information. The #1 language of the Internet is English, by a wide margin. In the chart below you can see that it is almost three times as prolific as the next language.
Global Internet Usage
Language - Number of users (millions)
English - 295.2
Chinese - 110.0
Spanish - 86.0
Finally, most programming languages are based in English. Their keywords, APIs, and documentation are mostly taken from English words. The number of mainstream languages that have non-English language equivalents are few and far between. With our languages already based in English, it makes sense to continue expanding our knowledge repository without dividing it into various inaccessible fractions.
Are there any exceptions?
Sure! On language-related sites, you are allowed to use those languages in addition to English. They are:
* There's a special site for full-Russian. See below.
Are there any Stack Exchange sites completely in other languages?
Yes. There are a few! While we are currently not accepting proposals for new non-English sites, these existing sites are available for anyone interested in those languages:
What should I do if someone else makes a post that is not in English?
If someone makes a non-English post on a site other than one in the above two lists, or in a language different from the site's accepted languages, first, check to see if it's spam. A small portion of wrong-language posts are actually spam, so be sure to check for that.
If it's not spam, vote or flag to close it as "Needs details or clarity" if it's a question, or flag as "very low quality" if it's an answer.
Do not translate wrong-language posts. Machine translations (e.g., Google Translate) can be inaccurate, and even human translations risk distorting the intended meaning of the post. It's up to the author to make sure that their post fits the quality standards of the site; if they don't, it reduces their chance of getting a good answer (in the case of a question) or that their post will be well-received.
Note that some sites may have different, possibly overriding guidance on moderating wrong-language posts. For example, Stack Overflow has its own community-specific close reason for wrong-language questions (but still otherwise follows the general policy above), and Mathematics allows for translation in certain cases.