There's tension here between faithful representation of the question matter and attracting answers. If you privilege faithful reproduction, you put the likelihood of answers at risk, and vice versa.
Let's say I write a question in, say, Spanish. Immediately I reduce my audience to the union of people who speak Spanish fluently and people who need to use Google translate. In the latter case, you ultimately risk faithful representation of the question matter.
The primary reason to avoid non-English questions is that unanswered questions are not desirable.
People who come to StackExchange want, and should get, answers. However, no other SE site allows questions to be substantially in non-English (except those specific sites for language learning).
I'm not entirely sure what would happen if you posted a non-English question here.
Perhaps it would be closed as off-topic, who knows. If it isn't it is extremely likely that it will go unanswered, unupvoted, and then deleted after a year automatically.
Even on language-learning sites such as English Language Learners, questions asking for the English equivalent about some "untranslatable" word in some other language gets closed - bear in mind, this is a site where we actively encourage non-English speakers to participate. Of course, this differs in that I imagine you would seek answers in the source language. But I don't think it differs enough.
Your example question
Without more information, I can't see why it can't be translated. Why is this untranslatable? Or is it just untranslatable by you?
To me, it seems like that question title would be translated as
What replaced "Title II: Regulation of the use of the title of legal advice" of "Act 71-1130 of 31 December 1971"?
Is that not a faithful translation? If not, why not?
Anyway, if it is a faithful translation, then the body of the text needs to provide more context if it isn't evident from the question title. In particular, we should avoid linking off-site to other resources without quoting the relevant sections, because we don't know how long it's going to be accessible there, and once the link dies, the link-only question/answer dies.
Splinter stacks for each jurisdiction
having better phrased questions, given such questions would produce pseudo-English. If the community choose to don't handle them at all, then the best would be to create a site for each non English speaking jurisdictions.
Personally, I don't think this is the way to go. Again, it's about getting answers, right? You could propose a La Loi Française site on A51, but I'd advise building a critical mass of experts here first, then doing so. Otherwise, such a proposal is unlikely to succeed.
Also, according to your comment, a French Law site is unlikely to be active because they can't ask questions to non-lawyers, apparently! Would this apply in other languages? How does asking it in French (or the other language) avoid this restriction (if it exists)?
And finally, after some reading, I discovered it is illegal for French peoples to ask written questions to someone which is not a lawyer (because it breaks lawyers monopoly to answer locals law legal questions). – user2284570 May 14 at 11:28
There's no point having a dozen localised Law sites, with three users each; if the desire for people to answer it is strong enough, surely they're going to find it on an English-speaking site, right?
I guess it depends on the nature of the questions, too, to some extent. If it's something that doesn't require statutory interpretation, the exact wording may not matter. If you're asking something about French legal history, it probably doesn't matter whether the translation completely carries the sense that the source text does.
The best course is to post the question in English but the body in the source language, and then post your best attempt at translation with it. This way, if someone who speaks the source language comes along and finds it, they can answer it. If no-one does, an English speaker can. And you get both: attempts from native speakers (possibly?) and answers from English speakers.
I also see no reason to object to the above suggestion, because you would be more easily able to gauge the potential interest in a language-specific law site.
I really can't recommend writing the whole question in a foreign language, but I wouldn't necessarily vote to close, if the attempt to translate it made it appear on-topic.