"Substantially different" is exaggerating it. Substantively, their dialect and standard English are entirely mutually comprehensible.
Secondly; if you'll notice, the NT was not translated into every variety of venacular Greek of the first century; the Vulgate was not written in every local patois of Latin. Historically, translations are made from one common dialect (usually the standard language or close to it). And the reason for that is that first, more people can use it that way, and second, there is a social perception of local dialects as less reverent and more informal--which makes them totally unsuitable for formal church use. This kind of linguistic activism is not done with liturgical sensibilities in mind, believe me.
There's another problem with using dialects like this, and that is their inherent instability. The hip-hop of today bears little resemblance to the African American dialects of the early 1900s--and we can predict pretty confidently that blacks 100 years from now will look at modern hip-hop and laugh at its quaintness and old-fashioned sound. Thus, even if it were advisable to produce a Bible or Mass in hip-hop, it wouldn't last 3 decades. (Imagine finding one in 1960's slang "groovy"..."that's a gas, man", and you'll easily get the point.) Slang goes from cool to corny really fast.
I think you make a good point in terms of reaching the people where they are at linguistically--however, doing it in the liturgy is not the way to go. Rather, the better thing would be to have the liturgy in a long-lasting, reverent language, and to allow priests in their homilies to adapt that to the idiom of the people. If it means using some hip-hop, so be it. As long as it is reverent, and as long as the liturgical language itself is not pegged to a slang that's going to be out of date 3 decades after it's introduced.
"Rather, the better thing would be to have the liturgy in a long-lasting, reverent language, and to allow priests in their homilies to adapt that to the idiom of the people."
Well they tried that and wound up with a Church almost universally ignorant of the Bible.
It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.
... not "out of the mouth of a priest".