Latin keyboard in the firmware on Sony PRS-T1 is not an Android one. It is a specially modified one and has some special integration with the rest of firmware. So deactivating it (albeit technically possible and relatively simple) will create unnecessary side effects. An example would be non-functioning (blowing) keyboard settings (in the settings screen). For this reason AMR does not allow you to disable or replace system keyboard application - the same as original firmware.
If you are willing to accept the consequences (not recommended) you need to physically delete Latin keyboard APK form system partition by moving it to data partition and changing its attributes using Total Commander or terminal. The rest you will have to figure out on your own