Unpivot avoid cartesian product on empty columns
This is what I came up with the help of reddit : 

SELECT DISTINCT reference, libelle, date_naissance, pays_naissance, nationalite, pays_fiscal, nif, us_tin, us_person, type_activite, etat_civil, rac, pep, departement, etat_individu, relation
FROM S2i_individu
LEFT JOIN relation_individu ON S2i_individu.reference = relation_individu.individu
CROSS APPLY (
VALUES
    (pays_fiscal_1,nif_1,departement_1,'t1'),
    (pays_fiscal_2,nif_2,departement_2,'t2'),
    (pays_fiscal_3,nif_3,departement_3,'t3'),  
    ('','',departement_4,'t4'),
    ('','',departement_5,'t5'),
    ('','',departement_6,'t6'),
    ('','',departement_7,'t7'),
    ('','',departement_8,'t8'),
    ('','',departement_9,'t9'),
    ('','',departement_10,'t10')
) AS v(pays_fiscal, nif, departement, suffix)
WHERE import_id = (SELECT TOP(1) id FROM import ORDER BY import_date DESC