Do be mindful of it, don't dumb anything down

Most non-native English speakers are much better at understanding texts than they are at writing them. So they will probably be able to understand your answer if you write it in non-simplified English.

That said, because many users of this site are non-native English speakers, I'd recommend not intentionally using complicated language. And I'd avoid uncommon words or slang terms that aren't related to the technology in question. For example, "dependency injection" and "debugging" are pretty obscure terms to most English learners, but should be familiar to most programmers.
But the same people won't necessarily know what words like "fastidious" or phrases like "to your heart's content" mean.

Not a native speaker? I am impressed. Most people born and raised where I live in the United States don't have anywhere near the lingual fluidity that you have displayed. To answer your question, most people I have met who are trying to learn English often are frustrated that everyone speaks to them in simple words and terms, because their goal is to learn the language to its fullest, not to get by with the basic points understood. Always answer as completely and accurately as possible, even if it means that the OP might need to look up a word or two. Chances are that some native speakers will need to look up a word too, if you answered as accurately as possible.

No

And I'll add another reason than the previous posters of negative answers:

Reading English is what developers do for a living. Being it MSDN, JavaDocs, this site or just normal books, we have a lot of practice doing that. Writing correct English is a whole different beast. That's hard because we never do that. It's rare that we actually speak English and that post on SO might be the only instance where the poster needed to generate proper sentences all by his own. So peoples' reading comprehension should be a lot better than their writing.

Don't expect people to not understand written English just by their proficiency to write English themselves. Even if they don't, looking things up in dictionaries is a lot easier from foreign to local than the other way round.

Simplifying is stripping information. I'm not here to lose information. So if we want to help non-native English speakers (and I'm one of them, so thanks in advance), we should write normal and above all correct English. That is really helpful.

NO (IMHO)

English is not my first language but I don't want/expect a "light" answer in SO because of that... and probably the other 99% of people that read the answer use English as first language.

I understand the intention and is nice :-) but the Lingua Franca of Internet is English. If one OP doesn't understand the answer... his responsability is improving his level.

Keep your answers as simply worded and structured as reasonably possible in general.

Your answers, while directed at the asker, are still intended to be generally consumable for others with the same issue. As such, you should configure your language to be as clear as possible to the general audience. Who the asker is should not factor into the structure of your answer.

You shouldn't dumb down answers just because you feel the asker has a weaker grasp of the language. Instead, you should strive to present the necessary content in the most easily-accessible and most user-friendly manner possible.

To that end, I argue shorter, more concise, more simply-worded answers are better than ones which are needlessly complex.

• They take less time to read and skim
• They are more likely to remain up-to-date if they have less extraneous information
• They will reach a wider audience no matter the asker's English level

I personally have a propensity to write long, rambling sentences with complex grammar, and I am certain that has hurt me here several times. I have also skipped answers (and questions) that may have been related to what I was doing because they gave me flashbacks to Tolkien's tree chapters.

So my advice, from personal experience on both sides, is keep it simple, period.

The best approach would be to do both. As the previous answers point out, your answer needs to be useful to all future viewers, not just the user who asked the question, so you need to include all the detail you can.

However, there is no reason why you can't either structure your answer so the the simple easy to understand points are made first. Or you could add a TL; DR summary at the end that you think a non-English speaker will understand.

Don't dumb anything down, even if you think it makes it easier to understand. It won't necessarily be simpler for the reader.

On simplifying language structure: Don't forget that there are automatic tools that are reasonable (at best, though improving daily) at translating correctly formed sentences. However try writing some form of pidgin in google translate, and pure gibberish comes out.

Whenever you're in the phase of struggling to express yourself in a new language, you already have decent comprehension skills. But nothing is harder to understand than over-simplified language. I would know, I've learned a couple languages myself, and what people remove to simplify language is what they, as someone fluent in that language, consider superfluous. However those can be important contextual clues to beginners -- especially from different cultures.

The same applies to individual complicated words: they can be looked up. If you replace them with a quasi-correct simpler word, your answer is less precise and it will be harder to understand, for the wider audience at least, and probably even for the OP.

Look at the Simple English Wikipedia front page (since you give it as example, emphasis mine):

This is the front page of the Simple English Wikipedia. Wikipedias are places where people work together to write encyclopedias in different languages.

The word place is overly dumbed down, and the description becomes vague. Calling it a website, and having people look that up if they don't know what it is, would be a better approach. Especially for SO where you expect content to be technically useful.

The only thing I think you may try to avoid are slang, idioms, or obscure words (as covered by KWeiss).