PARIS — David Trueba’s “Soldados de Salamina” (Salamina Soldiers) has joined the Cannes Film Festival’s Un Certain Regard lineup.Most of the sidebar was announced by the festival’s artistic director, Thierry Fremaux, last week. “Soldados” bowed in Spain in March to excellent reviews. It has notched up more than $1 million at the box office. Toplining Ariadna Gil and Ramon Fontsere, pic tells of a journalist who investigates the mass executions of Franco sympathizers during the Spanish Civil War. Trueba’s “La Buena Vida” screened in the Cannes Directors Fortnight sidebar in 1997. This year’s Cannes fest opens May 14.