"Proper" has several meanings and usages. For whatever reason, with this particular usage, it's common to put it after the noun it modifies. It's covered by definition 6 in Merriam-Webster:
6: strictly limited to a specified thing, place, or idea: "the city proper"
Or in Cambridge:
[ after noun ]: belonging to the main, most important, or typical part:
"It's a suburb of Los Angeles really - I wouldn't call it Los Angeles proper."
The point here is that it's for times that we're talking about what might be included in some category, and distinguishing between official or strict inclusion and more unofficial or general inclusion. Note that both dictionary examples talked about cities. This is perhaps the most common situation for it: A house might be outside the city limits, but still be in the general metropolitan area. (E.g. the opposite to "Atlanta proper" might be "greater Atlanta.") In your example, the author is distinguishing between a language that is "half-Magadhi" and the language that falls within the most official or strict boundaries of Magadhi—as it says earlier, "canonical."
Note, this ngram shows that this "after the noun" usage isn't terribly common, and that by far the most common uses for it are about geographical boundaries.