SAN SEBASTIAN — Elena Anaya (“The Skin I Live In”), Valeria Bertuccelli (“A Boyfriend For My Wife”) and Esteban Lamothe (“The Student”) will topline “I Thought It Was a Party,” directed by Argentina’s Victoria Galardi (“Mount Bayo”).UIP will release “Party” in Argentina. From a Galardi screenplay, “Party” makes its international bow at San Sebastian’s 1st Europe-Latin America Co-production Forum, which kicks off Thursday at the Spanish festival. A dramedy, “Party” turns on Ana (Anaya), who starts a passionate relationship with the ex-husband (Lamothe) of a close friend, Lucia (Bertuccelli), while house-sitting for her and looking after her daughter. Lucia has no right to feel betrayed, but she does. Gale Cine, Galardi’s label, produces with Magma Cine, whose credits include Pablo Fendrik’s two Cannes Critics’ Week entries, “The Mugger” and “Blood Appears,” and Israel Adrian Caetano’s upcoming “Mala.” The producers are in conversations with a potential international co-producer, said Nathalia Videla Pena, who produces for Magma. They aim to shoot from December in and around Buenos Aires. Galardi’s films — “Bayo,” debut “Lovely Loneliness,” co-helmed with Martin Carranza — are distinguished by irony and quiet psychological observance. Turning on “friendship, ageing, vanity, and the search for love or its lack,” “Party” delivers “an ironic take on human bonds, the thin line that sustains relations between a couple and between friends,” Videla Pena observed. She added that it is also a step-up for Galardi, in its greater maturity, the Argentinean distribution commitment, international cast, and possible international co-production.