Apart from argumentation on Chinese language and Writing:
vereinfachte chinesische Sprache
Do not have much semantic difference to me (in your specific example, but see below). Connecting the two adjectives (they definitely are adjectives) is common, but not always accepted practice in German - Especially, when there is no obvious need to do so, you have found a completely acceptable alternative using your second example.
Using more than one adjective with one substantive can transport two distinct meanings:
The second adjective is used as an unrelated additional attribute to the substantive, thus extending the first one and only tied to the substantive
The second adjective is used as a modifier for the first one, so it is bound to the first adjective
While the hyphen form could imply (2) - closer bound to the other adjective, I am not sure that is always the case.
die helle blaue Blume
That definitely says the "Blume" is "hell" and "blau" and not "hellblau" could be expressed as
die hell-blaue Blume
according to your example. Here it is not very clear whether hell applies to the Blume or to blau. It could also mean
die hellblaue Blume
So it is preferable to use the non-hyphen form, because that makes it much more clear.
Transferring back to your example, vereinfacht-chinesische Sprache could mean "die Sprache ist vereinfacht und chinesisch" (unrelated) or "Die Sprache ist chinesisch in vereinfachter Form" (modified) - In your example, there is maybe little to no difference to me (but I don't know Chinese...)