See below for information on pre-1778 translations of Voltaire’s works into different languages:
Dutch and Flemish
Jeroom Vercruysse, ‘Bibliographie provisoire des traductions néerlandaises et flamandes de Voltaire’, SVEC 116 (1973), p.19-64.
English
Hywel Berwyn Evans, ‘A provisional bibliography of English editions and translations of Voltaire’, SVEC 8 (1959), p.9-121.
German
Hans Fromm, Bibliographie deutscher Übersetzungen aus dem Französischen, 1700-1948 (6 vols, Baden-Baden: Verlag für Kunst und Wissenschaft, 1950). See also this online resource.
Italian
Theodore Besterman, ‘A provisional bibliography of Italian editions and translations of Voltaire’, SVEC 18 (1961), p.263-310.
Portuguese
Theodore Besterman, ‘Provisional bibliography of Portuguese editions of Voltaire’, SVEC 76 (1970), p.15-35.
Scandinavian languages and Finnish
Theodore Besterman, ‘A provisional bibliography of Scandinavian and Finnish editions and translations of Voltaire’, SVEC 47 (1966), p.53-92.
Spanish
Francisco Lafarga, Voltaire en Espagne (1734-1835), SVEC 261 (1989).