Transition level has to be readback and what about transition altitude? is it also required to be readback by the pilot?
The word “level” is used to refer to Flight Level (or Pressure Altitude). It can (and probably is in this case) also infer the word Altitude (or True Altitude). They are sometimes used interchangeably. Though, by strict definition, they are not the same. 

In the US, QNH (Transition Altitude), QNE (Transition Level), and QFE (Transition Height) are not widely used much outside of the military. Instead, the term Lowest Usable Flight Level is substituted. The word “Transition” itself is commonly used to refer to passage through an area. The process to transition the Transition Layer between the Transition Altitude and the Transition Level is standardized for all flights and airspaces.

The Flight Level is the altitude standardized at an altimeter barometric pressure setting of 29.92 inches of mercury. That allows all aircraft to fly at a standardized altimeter adjustment. In theory, this should avoid confusion caused by aircraft flying at different Indicated Altitudes but the same actual True Altitude due to different Kollsman Window settings. All aircraft in the US are to adjust their altimeters and fly using Flight Levels at and above 18,000 feet MSL. Above this Flight Level, precise Absolute Altitude is not necessary. 

The following was taken from the FAA-H-8083-16B, Instrument Procedures Handbook, 2017 (IPH)