Luxembourg general election: October 14, 2018

[Josecardoso17]

Finland has first socialist Prime minister among EU countries, but only two times left-wing majority in parliament since independence, last time 1968.

Wrong. Poland, Germany and Austria had their first socialist/social democratic head of government in 1918 (Ignacy Daszyński, Friedrich Ebert and Karl Renner respectively).

I assume you were referring to Tanner, who only became PM in 1926. And it's not the first among Scandinavian countries either (see Hjalmar Branting in 1920).
 

Even friggin Ramsay MacDonald had been earlier than Tanner.

Let's just do a recap of first socialist/social democratic/labor heads of government of EU countries:

Austria: Karl Renner (1918-1920)
Belgium: Paul-Henri Spaak (1938-1939)
Bulgaria: Andrey Lukanov (1990)
Croatia: Ivica Račan (2000-2003)
Cyprus: Never
Czech Republic: Miloš Zeman (1998-2002)
Denmark: Thorvald Stauning (1924-1926)
Estonia: Otto August Strandman (1919)
Finland: Väinö Tanner (1926-1927)
France: Aristide Briand (1909–1911) or Léon Blum (1936-1918) [Briand may be considered a socialist/social democrat by some measures, but Blum headed a decisively socialist party]
Germany: Friedrich Ebert (1918-1919)
Greece: Alexandros Papanastasiou (1932)
Hungary: Miklós Németh (1989-1990)
Ireland: Never
Italy: Ivanoe Bonomi (1921-1922)
Latvia: Never
Lithuania: Adolfas Šleževičius (1996)
Luxembourg: Never
Malta: Paul Boffa (1947-1950, while still under British rule) or Dom Mintoff (1971-1984, after the independence)
Netherlands: Willem Schermerhorn (1946-1947)
Poland: Ignacy Daszyński (1918)
Portugal: A bit crazy due to earlier parties, so I'll go with Mario Soares (1976-1978)
Romania: Nicolae Văcăroiu (1992-1996)
Slovakia: Robert Fico (2006-2010)
Slovenia: Borut Pahor (2008-2012)
Spain: Manuel Azaña (1931-1933)
Sweden: Hjalmar Branting (1920)
United Kingdom: Ramsay MacDonald (1924)


I probably missed something, so please feel free to correct me.

Vasco Goncalves certainly has a better claim than Mario Soares for Portugal.

nope