Which licence choose for a new academic computing language?
Oh, no, yet another format!

Now seriously, you can legally register a format, but I don't see a big benefit from doing it.  For what I see, licensing is essentially a copyright of the actual text. In most practical purposes, having a public, freely available document explaining it, should suffice.

To support my feeling, I have been digging in some academic examples I am familiar with and they don't specify any.

If you want to be sure, you could see what other people have done, for example, OpenDocument format.