I did not find a meta for law. Either I'm blind or it does not exist for betas or something.
On Stack Overflow, you are required to write in English. There exists Spanish and Portuguese versions, but on the main site, English is required. Always.
My questions is fair and simple: How is it on law.stackexchange.com? Are you allowed to use language X if the question is about the law in country X?
I would argue yes. After all, most people qualified to answer a question with the tag [sweden] will be native Swedish speakers or at least speaking fluent Swedish. The same goes for most people who have a genuine interest in the question.
Another thing is that law has a lot of hard terminology. The same goes for technology, but there English is the standard. If you speak English fairly well, you will have no problem at all to post questions or answers at Stack Overflow. But most Swedish people would find it very hard to translate legal stuff to English, even though we are quite known for being good at it.