It was partially trained on EU and UN documents. Interesting observation, less emphasis on parliamentary/legal data vs a data set like OpenSubtitles, which has more casual language use, might give a better performance.
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."