The code "nah" does not refer to a specific language, but to a collection of languages, namely the "Nahuatl" languages.
This code corresponds to an existing Wikimedia project, which can be found at b:nah:.
This Wikibooks project existed in a subdomain but is now closed and locked. There does not appear to be any meaningful content on any of the five mainspace pages there.
Because this language code is actually a code for a collection of languages, it would be preferable to start a new project under the language code of an individual language within this collective. See Category:Incubator:Nahuatl languages for more details.