We all know that she can't use magic but she can speak. she can easily tell the story to the Asta & others about what happened. she can easily tell the truth to the current wizard king who can easily find out everything or maybe he can turn back 1st wizard king and Nero back to human form.
Nero was not able to speak before the elves re-incarnated.
Edit: Actually it wasnt until the shadow palace was summoned at Clover Castle. This is from the Wiki
> When Asta asks if she is Nero, she confirms and explains that she was originally human and could not speak as a bird until the Shadow Palace was summoned