In the first season installment "Divorce Isn't Everything", Mary mentions that she can't speak French but can speak Spanish. Later in the series, while at a Mexican restaurant, she indicates that she can't read the menu because she took French in college.
Exteriors of Mary's residence show that the apartment with the balcony (which would be Mary's, based on the interior shots of her apartment) is on the top floor, yet Rhoda's apartment is upstairs from Mary's.
On the newsroom wall there are various world clocks like you would see in a typical newsroom of that era. In the center is the Minneapolis/St. Paul clock, and to its left is a Chicago clock. Minneapolis and Chicago are both in the central time zone, so there is no need to have two different clocks. Furthermore it can be seen that the Chicago clock is set roughly an hour ahead.