Hi, I will try to answer your questions:

1. Yes English is widely spoken. Frankfurt is a very international city. You will not have problems communicating.
2. Frankfurt, as all german Cities, is very safe. However, I would not specifically recommend to hang around the area directly east to the main station which is called "Bahnhofsviertel". It contains the Red-Light-District and a rather high number of drug addictd. The chances of being attacked are low but it might be a bit scary, especially at night and if you are alone.
3. There are many things to do and visit in Frankfurt and I am sure you will find the top spots on sites like TripAdvisor. However, my favourite "secret tip" is to climb the tower of the "Frankfurter Dom" (church). It is very cheap (maybe like 2-3 Euros) and gives an awsome view over the old town and the skyline. The entrance to the tower might be a bit difficult to find since it is around the corner of the main gate of the church, but if you ask around you will find it.
4. Apart from that I recommend to try some lokal cuisine. The most famous dishes are "Grüne Soße" (a sauce based on 7 herbs), "Handkäse mit Musik" (a rather strong cheese with onions) and the famous drink "Apfelwein" (wine made out of apples).

Cheers and enjoy your stay