Was the model trained on a corpus of EU documents? I'd imagine that's a great source of multilingual training data, but could lead to an obsession with the EU in the translations
The word "Formell" seems to trigger the EU focus, maybe it's very characteristic for established norms within the EU's translation service. Changing it to the closely related "Formal" makes the translation work: "The law on women's voting became effective on 16 March 1971." Another option would be "Offiziell", which gives the so-so: "Women's rights were officially enforced on 16 March 1971."