It’s not hard and doesn’t need language awareness; I described how to detect it: if there’s no space before an end-of-line em dash, suppress the segment-break-replacing space.

There seem to be some locales or styles that use asymmetric spacing. From the Zen of Python—note different spacing based on context and position within the sentence:

    There should be one-- and preferably only one --obvious way to do it.
    Although that way may not be obvious at first unless you're Dutch.
    [...]
    Namespaces are one honking great idea -- let's do more of those!