I just went to the station and checked. English is the only foreign language offered on the machine. You can select it in the lower left corner of the starting screen. Then, select 'buy separate ticket'.
You can buy a ticket for a date in the future. Simply select day return or single on the left, enter your destination and travel class. Then you get the choice between 'valid today' and 'other travel dates', up to a week in advance.
And yes, unless you have one of the home printed tickets like described above, you are required to tap in and out with a disposable ticket from the machine or counter. Otherwise your ticket is not valid even though it is dated, routing specific and prepaid. Weird but that's how it is.
Tapping in and out can be at a yellow NS pole (hold it against and wait for the beep). Do not tap a red pole, that is for another service.
These poles are located on the platform or entrance at smaller stations. At Schiphol they are everywhere in the central hall and before you enter the escalators down to the platform.

At larger stations, you may need to tap in and out at the gates.
Use the scanner on the right of each gate. At some stations, like Amsterdam Central, the gates are always open. At Leiden they are always shut. This is why it is important to also tap in, otherwise the gate will not open.