Salaj was born in Valpovo and received secondary education as a tailor.
In 1907 he travelled to Austria, Germany and Switzerland looking for work, and became a social democrat and a trade union activist.
In 1909 he moved to Sarajevo and became a local sewing trade union representative.
Between 1930 and 1944, he was the representative of the Communist Party of Yugoslavia with the Comintern, stationed in the Soviet Union.
In SFR Yugoslavia, he was decorated with the Order of the Hero of Socialist Labour.