I see there are rules against other language use. I think it would serve the international character of this site well to allow QandA in languages non english speakers can understand. I do not see how anyone would loose anything by allowing this (e-resources are practically unlimited) and there would be a lot to win. We have google translate, for instance.
Your question seems to pertain to whole texts in other languages, so I'll first answer about that.
The whole site is in English, such that the texts of both questions and answers as a whole have to be in English. In the end, you are writing for your readers, and you can only be certain that they know English (at least to the point that they are willing to use an English-language site), but not other languages. So, writing an answer in French is considered "not an answer" and removed.
The international thought is already being lived on Stack Overflow, our flagship site, which is about computer programming. Beside the Stack Overflow in English, there is a separate Stack Overflow in Russian, one in Portuguese, and others are being prepared. But note that almost all people are comfortable in a single language only, and even multilinguals don't benefit of mixtures, so these are separate communities.
You also posted a question on main, Substitution for hagelslag (chocolate sprinkles for bread), which may have been the trigger. In this case, it is not a whole question written in Dutch, but a question in English which refers to specific culinary terms in Dutch.
Such embedding of specialized terms is OK. Sometimes, it is even necessary - English has no words for many foods such as or lavash or gulab jamun. In other cases, English might have a decently fitting word, but you as the OP might be interested in information about a country-specific version. Asking something about gravlax can lead to different answers than asking about cured salmon in general. In yet other cases, English has adopted the term to the point it has become an English word of foreign origin, such as mozzarella.
In these cases, you have some leeway and have to make a decision. The foreign term can be more precise, but you have to accommodate your readers. The less common the term in English, the more likely it is that you will confuse them, rendering your post pointless. So you need to use foreign terms judiciously, and it would be best to also provide an explanation for them in the body of your post.
A post which is written in English but employs foreign terms will not be deleted. But other users who have trouble understanding may edit it to use English terms, especially if no meaning is lost by the translation. This is considered a valid edit, as it improves the understandability of a post. If a post employs foreign words to the point that it is impossible for an English speaker to even get the gist of it, it might get deleted for the "very low quality" reason if an answer, or the "unclear what you are asking" reason if a question. This refers to posts of very mixed language - a few terms are OK, especially when information would be lost in translation.
tl;dr there are a lot of serious problems with allowing other languages, so as wonderful as it would be if the site could cover all languages, it's just not something we can really do.
We'd lose something pretty big if we allowed other languages: a lot of our content would end up much less useful.
English is by far the most common language on the internet. Even if it's not the first language for most of those people, it's a common language. Writing posts in English guarantees that the largest possible number of people will be able to read them. That makes our content more useful - and it makes it more likely for questions to get answers. If those same people instead posted in a foreign language, their post would be readable and useful for a much, much smaller number of people.
We're always happy to provide assistance if the language barrier is an issue - people can just try their best in English, and let us know that it's not their first language and they could use some help cleaning things up.
Yes, this is unfortunate for people who don't speak English at all. But we can't be all things to everyone. We've built a site that's useful to as many as people as possible. We would love for it to be useful to everyone, in all languages, but unfortunately that's just not feasible. Not everything else people want can reasonably be added - we're not covering health, we're not covering gardening, we're not covering how to run a restaurant, and we're not covering other languages. Yes, those are all totally reasonable things to be interested in, they're just not things that are part of this site.
The giant issue above (less useful content) is definitely reason enough to build just an English site. But even if we somehow changed to a rule where you post in English if you reasonably can but are allowed to post in other languages, so that we were not actually losing any potential English content (this sounds really hard to pull off, but let's pretend) there'd be a lot more issues, for example:
- we wouldn't have any way to filter the question lists down to things readers can actually understand
- we wouldn't have many users who spoke that smaller language, meaning fewer answers, less voting, less sense of community, and likely lower quality posts and less reason for users to stay
- moderators wouldn't be able to do their job (meaning more spam, more hostile discussions, more low-quality or non-answers, and so on)
- meta discussions would be hopeless - just imagine a Portuguese-only user coming to meta today
So what can we do? If there's not enough users in a given language interested in a cooking StackExchange site to form a community, it's just not going to work no matter what you do, so there's not much sense worrying about it.
In cases where there are actually enough users (or potential users) to support a non-English community, it's far better to have a separate site as has been done with the StackOverflow offshoots. Everyone who visits the Portuguese site speaks Portuguese, the moderators speak Portuguese, and it's a clear community in its own right. If there were enough support, you could by all means make a Dutch or French cooking site.
I have my doubts about whether there would actually be enough users, but you're certainly welcome to go to area51 and give it a shot. Note that you really do need a lot of users - I believe it takes at least 80 users to have enough votes on sample questions to finish the definition phase, and 200 users to commit to joining the site.