If you do a search, you will find that it is spelled both ways (by evos and creationists alike). Indeed, the valley Neandertal was named after is spelled Neandertal, not Neanderthal.
Neanderthal is English, Neandertal is German.
1.William King was the first to make a taxonomic name for the group we call Neandertals. He named it Homo neanderthalensis — that’s “neanderthalensis” with a “TH”.
It was Neanderthal Valley before the Germans dropped the 'h' in 1901!