Fläder, Sambucus nigra, is ‘elder’. The Swedish word comes from Low German vleder with the same meaning, but current German Flieder means ‘lilac’, Swedish syren, Syringa vulgaris. ‘The German word for elder is Holunder, related to Swedish dialect and Norwegian hyll, Danish hyld and Old Swedish hylle. I thought those might be related to English holly, but found no indication to that.
English elder is also of unknown origin, possibly related to alder.