Hi!
We will want to distinguish "a known language not on this list (mis)" from "an unknown language (und)" and "translingual" (Wiktionary uses "mul" for translingual, but that's not technically correct).
I think "mul" is for "text in more than one language" and there's also "zxx" is for "text that is not defined as being in any language at all".
BTW, BCP 47 also says:
The 'mis' (Uncoded) primary language subtag identifies content whose language is known but that does not currently have a corresponding subtag. This subtag SHOULD NOT be used. Because the addition of other codes in the future can render its application invalid, it is inherently unstable and hence incompatible with the stability goals of BCP 47. It is always preferable to use other subtags: either 'und' or (with prior agreement) private use subtags.
So maybe using und would be a good idea.