The following European parties claim to be SDs:
Spain: Spanish Socialist Workers' Party (PSOE), Eusko Alkartasuna (Basque Country)
I would consider the bolded ones SDs and the crossed out non-SDs, but there are a lot of Eastern European parties where I am in doubt (left populist, ex-communist, tools etc.) and a lot I do not know enough about, then there are the left nationalist. Not easy to draw the line.
You should add the Republican Left of Catalonia to the list. They define themselves as SD and have enough relevance. It's hard to draw a line, indeed.