This context comes from the book "Black Rednecks And White Liberals" By Thomas Sowell.
Being part of the Roman Empire meant that Western Europe had not only a common language—Latin—but a literate language, centuries before there were written versions of the various Slavic languages of Eastern Europe.
Definitions I found on Merriam-Webster:
EDUCATED, CULTURED
literate executives
versed in literature or creative writing: LITERARY
literate novelists
LUCID, POLISHED
a literate essay
Only the last definition describes something other than people but I somehow don't feel like this is the correct one. Off the context, it seems like it means "written language" but I can't seem to be able to find an appropriate definition.
I know that literate is also a noun that means:
(Merriam-Webster)
2 : a person who can read and write
So maybe "literal language" means a language for a person who can read and write?