Fix i18n translation issues

Fixing up some strings that were improperly i18n'ed or caused issues that the translators brought up.
In tests, `MIN_DIFFERENT_STUDENT_PASSWORDS_BEFORE_REUSE` is set to 1. And there's no verification disallowing it. Plus, different languages have different pluralization rules, where "2 passwords" and "3 passwords" have different pluralizations. So I think it makes sense to use `ungettext` here.