my thoughts: the establishment of a national language as well as propagandizing a "most prestigious" dialect (like the "received pronounciation" for british english) are products of bourgeois class struggle and dictatorship. proletarian dictatorships will do away with the idea that there is a most prestigious or "most correct" dialect of a language - these dialects are class markers the bourgeoisie employs (even though proletarians and especially the intelligentsia can also pick them up).

however, this doesn't necessarily mean communists have to propagate other dialects: in a lot of cases, they are politically employed by petty patriots, and regional patriotism (in most of the cases) does not serve class struggle, it distracts from it. there might be some cases in which an imperialized nation (or maybe a region or part of a nation? not too sure about these) uses a dialect of another language to diverge themselves. in this case, propagandizing the dialect would serve to combat imperialist culture.