Should we adopt a standarized way to indicate that we're writing a sentence that is incorrect?
Yo soy bien.(I've used it in in this answer)
- *Yo soy bien.
- ×Yo soy bien.
The first example makes clear the part of the sentence that is incorrect. The second example would make sense to linguists, but others would probably go looking for a footnote for the asterisk at the bottom of the question. The third example could be interpreted as a bullet point, or just referring to the first word and not the whole sentence. The first seems like the best option to me.
In general though, I'd say people should use whatever they're comfortable using. This seems too specific of an issue to solidify a standard and go around editing posts and policing people's formatting.
It probably depends on context.
When quoting something that is ungrammatical, use '(sic)'
In your example, the context itself I think makes it clear, as you say:
You can say "lenguaje no verbal", "lenguaje de los ojos" but not "idioma/lengua no verbal", "idioma/lengua de los ojos".
So I don't think any additional markup is necessary in that case (and in fact, I think the additional markup reduces readability)
You can always use an asteriks* or other note1 and explain your meaning later.
*I intentionally spelled asterisk incorrectly so I'd have an excuse to demonstrate my meaning.
1Others may prefer numbered references!