Although it does not make sense, entering both will not hurt, and may help the disabled. It is important to correctly declare the correct language. Why? Well, the disabled use various software to read the text. The software is different – one program recognizes "lang", the next "xml:lang", and another recognizes both.
So typing both IMHO is not a bad idea.
But I know that there are people here who have the opposite opinion.
http://kb.daisy.org/publishing/docs/html/lang.html