I believe fossfreedom's answer to Does it make sense to flag non-English questions? is our standing policy on what to do--or, at least, whether or not to flag--when someone posts a question in a language that is not English. This supports the message you got from a moderator who declined your flag. As that post says:
TL;DR: Do not flag language issues. Translate them yourself.
It's been almost five years since that was posted, and in case the community ends up wanting to adopt a different consensus, I'm posting this answer, focusing on the history of our policies for non-English posts, rather than voting to close this question as a duplicate of that one.
That policy postition was previsaged by Oli's answer to What to do with non-English questions or answers?, which is currently the master question against which several related meta questions1 have been closed as duplicates.
On the other hand, several of the answers to Please ask people to translate, don't just close a question, including terdon's and Tim's, which are pretty highly upvoted, advocate closing such questions as unclear.2
Based on that, it seems to me that it is currently considered entirely acceptable (though not mandatory) to vote to close a question in a non-English language as unclear, even if it does not have any other problems and even if there is no reason to think it cannot be translated quickly and easily. However, if it is subsequently translated, then it would be eligible for reopening provided there's no other reason to keep it closed.
But this still does not require a custom moderator intervention flag. Users with enough reputation to cast close votes could cast their vote, and others can flag for closure--though those flags may be declined (semi)automatically if the post is subsequently translated and consequently receives no close votes.
(As for how best to present a translation that one has produced or obtained, gertvdijk's answer to If a question is written in a non-English language with an English translation provided should the non-English part be removed? seems to be our current general consensus.)
1 Other well-received meta posts that have supported this view include Rinzwind's answer to How to prevent answers and comments to question in non-english before they're translated?
2 See also Bruno Pereira's answer to this older question. It doesn't have links to the discussions it refers to, and I'm not convinced it adequately captures what the "extended discussions" from around that time decided on. But it's pretty highly voted, which counts for something. This meta question by Jacob Vlijm (and Elder Geek's answer) also expresses the view that translations shouldn't be expected.
My own personal opinion about what our policy should be is that, in cases where it is reasonably easy to do so, it is better to translate before closing a question rather than after. I believe this can help figure out if there are other problems with the post. I also believe that it is more likely that someone will be able to recommend other help resources in the OP's own language, if they have some idea of what the question is about. (Of course there are exceptions. Sometimes something is clearly low quality for other reasons, or clearly spam, and it would be pointless to translate it. In the spam case, worse than pointless.)
As dessert points out, there's a policy about posts in non-English languages. However, I think it is acceptable for us to approach the issue differently if we choose.
It is extremely far from clear to me that this policy is even intended to be network-wide. Jeff Atwood's blog post is about the original three sites. It is based on a meta answer that mentions Stack Overflow and no other site. There are localized Stack Exchange sites in other languages now, at least for Stack Overflow, which doesn't challenge that policy for English language sites, but does serve to show how much has happened since then.
More important, though, is that the communities on individual Stack Exchange sites (like Ask Ubuntu) are free to make our own policies on most things.
One key point in the Ubuntu mission is that users, "Should be able to use their software in the language of their choice." I do not think this obligates Ask Ubuntu to accept questions and answers in all languages (though I do think it underscores the importance that resources for different languages be fostered throughout the Ubuntu community even if not on Ask Ubuntu). However, I do think it means that we should be more accepting of questions that are not originally posted in English than other English language Stack Exchange sites might be. Although it's not for everyone and it's not best for everything, there are major advantages to the Q&A format we use here on Ask Ubuntu, and nobody knows when, if ever, there will be Ask Ubuntu sites in other languages.
On the one hand, I do give some weight to the point that if people can't ask a question in English then how are they going to understand an English answer. On the other hand, as Oli's post has mentioned, machine translation often works acceptably in both ways (though, as Mateo has pointed out, not always, and even in general this isn't a topic everybody agrees on). Users who ask here often have tried or at least considered other sites. And even in cases where the OP cannot use an answer, it may have value for others and thus contribute to the goals of our site.