Why is the main site entitled "Chinese Language" rather than "Chinese Languages"?
I am sure the same argument could be said of English, French, or any other language. Regional variances, derivations, and topolects are not uniquely Chinese in nature (e.g. American/Australian/UK versions of English,...). I doubt you are recommending that all sites be renamed but the argument you've given could very well use that justification for what seems a questionable inference of semantics regarding the pluralism of the word "language". A word by the way whose definition itself implies some understanding of the implicit variations of which you complain.
(just to recap the definition of the reported problematic word)
language |ˈlæŋɡwɪʤ| noun 1 the method of human communication, either
spoken or written, consisting of the use of words in a structured and
conventional way: a study of the way children learn language | [ as
modifier ] : language development. • any nonverbal method of
expression or communication: a language of gesture and facial
expression. 2 the system of communication used by a particular
community or country: the book was translated into twenty-five
languages. • Computing a system of symbols and rules for writing
programs or algorithms: a new programming language. 3 the manner or
style of a piece of writing or speech: he explained the procedure in
simple, everyday language. • the phraseology and vocabulary of a
certain profession, domain, or group of people: legal language. •
(usu. as bad/strong language) coarse, crude, or offensive language:
strong language. PHRASES speak the same language understand one
another as a result of shared opinions or values. ORIGIN Middle
English: from Old French langage, based on Latin lingua ‘tongue.
("language", New Oxford American Dictionary)
Are proposers of this site suggesting that there is only one single "Chinese language"
No, I don't think so, and I'm not sure why anyone would make this assumption.
So to recap, I think that "they", whoever built the site, got the name right.