What for example should we do if the kid doesn't speak English as a first language?
Identify the language the child is speaking and get an interpreter? Hospitals and police forces can and do provide interpreters. Schools in areas where many children speak languages other than English often have staff who speak the local languages.
We won't try and unpick what they are saying because it's not the word vulva?
If a language other than English was spoken, then none of the other words would be understood either.
...or if they use more common slag like lady bits?
It's the very uncommon slang, the 'family language' terms, passed down through the generations, that cause huge problems.
But there is a problem with 'lady bits', 'front bottom', too; nobody knows if this refers to vulva, clitoris, or vagina.
Would we dismiss a boy who says X touched my dick or my pee pee because they didn't use the word Penis?
But those terms are very clearly referring to the penis. Girls have more genitalia, and the fact that some are internal makes a difference in legal terms.
It is very important for a girl to know that she has a vagina and to know what it is called.