In Swedish, "Sydsverige" often just means the very southern tip with Skåne and Blekinge. I never hear people say "Stockholm is in 'Sydsverige'" in Swedish. But

In Swedish, "Sydsverige" often just means the very southern tip with Skåne and Blekinge. I never hear people say "Stockholm is in 'Sydsverige'" in Swedish. But that wasn't the context in this case. I described the situation in Sweden to a person from Germany. In that context, I think it is reasonable to call the southernmost one third of Sweden "Southern Sweden". It will give him the correct mental image of what I meant when he looks at a map of Sweden.