Dataset is new to me as well. I was a little surprised that DateListPlot was happy to ingest Dataset of GroupBy. My back up plan was to GatherBy the country.
b = {#[[All, 2 ;; -1]], #[[1, 1]]} & /@ GatherBy[Normal[Values[a]], First]
This new structure { timeseries, legend} can be used two ways, PlotLegends option method:
DateListPlot[#1, PlotLegends -> #2] & @@ Transpose[b]
Or the Legended method:
DateListPlot[Legended @@@ b]
