I suspect that “Londonderry/Derry” could be considered a single name at this point. There’s a reason people call it “Stroke City” but don’t call Brussels the equivalent of “Spaced Dash City” in its local languages. (Though it sounds like “Squiggly City” might be a possibility too…) If that means a renderer might label the city redundantly, at least there’s an interesting explanation for it.
Aside from that special case, you’ve hit upon the point that keeps getting missed in this thread: that language is not the only reason for a name or *_name or name:* tag to contain multiple equally valid values. Sometimes we can differentiate with subkeys, but just as with linguistic differences, there still needs to be a name to “split the difference”, so to speak. Other times, there’s really no reasonable way to qualify which name is used in which context.