This is a world history class, second half--you're right about earlier imperialism. I think that too much of the early modern and modern literature still uses the middle ages as an undifferentiated "other" (usually a bad other) against which to contrast modernity, and doesn't go much beyond that blunt comparison. Medievalists have done good work in discussing premodern colonialism (Ireland, anyone?) and imperialism, as have classicists (Alexander, Rome ,...).

I also have a variant of this discussion in the Western Civ II class, where I let them deconstruct Rudyard Kipling's "White Man's Burden," and then give them at least two of the multiple verse responses to it. The byproduct of imperialism, of course, is that far more people have read "The White Man's Burden" than have read Edward Morel, H. T. Johnson, or the countless editorials and commentaries that followed on the heels of Kipling's poem and nailed its essential hypocrisy (even if Kipling meant it as a savage satire, as some have suggested, it was taken by many readers at face value).

I think your point about language is an excellent one, and dead on about the varieties of English and Spanish. (Add regional dialects within countries, too, and I would also add "Southern communities ... " to the list, and perhaps, "some black cultures ... ") I've had students of all races speak in pure South Georgian, and students of all races speak in "standard" English, depending on where they were from.