The official faq Do posts have to be in English on Stack Exchange? on mother meta refers to the official blog entry on the topic. Note that the focus on "programming questions" is because back then there were only SO, SU and SF around, but the essence still is valid:
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 a corollary, questions not in English will be closed. Questions that include both an English version and the original non-English version are fine if the asker is uncertain about the correctness of their translation, but this should be the exception rather than the rule, and in particular all further communication, in particular answers, should be in English. Questions in bad or broken English are also completely fine - they should be fixed on sight by more fluent speakers, but it is rare that mere language issues make a question so incomprehensible that it would need to be closed.
In the end, I oppose the posting of non-English language posts without an accompanying translation attempt, and prefer broken English over a non-English language post because using more than one language within a community splinters said community into smaller camps of users who share a certain language. Things like the review system only work because all users can communicate with each other and the idea of meta discussions (such as this one!) is wholly absurd if we do not expect everyone to speak the same langauge. And everyone able to follow the average meta post should also be able to make an intellegible attempt at writing a question in English, no matter its actual grammatical correctness.
Finally, content needs to be moderated. When a question is posted in a language I do not speak, how am I, or any other moderator or reviewer, supposed to decide whether the question is on-topic, or, worse, whether it even is a physics question and not just spam or some abusive rant? Wait for someone who actually speaks the language? How does one find such users? From this point of view, it's completely impractical to allow non-English content while upholding our quality standards and the idea of forming a single community.