It bothers me that you cannot stop the Arduino from executing the program. Once you've typed in a text,
it translates and blinks, but you cannot make it stop, e.g. in case you've changed your mind about the message you wanted to send.
Arduino is meant to run forever, at least as long power is applied. It never finishes.
Perhaps you program it to do nothing visible to the outside any more, until you press the reset button, but that's just one possibility to pass the time until power goes off.
You upload your eternal sketch once, and then it starts...
Usually it waits for events on input pins, or for data to arrive on Serial, and handles that.
E.g. it reads a character and translates that to morse code. When done, it reads the next.
If there's nothing available, it simply repeats trying to read.
That's why this function is called loop().
Once you're happy with that, you can think about more sophisticated buffering ( in case you type faster than the morse sequence takes )
(By default, you might notice that Serial buffers about 64 characters automatically)
After you got your self controlled buffering fine, you can think about interrupting and breaking a pending buffer sequence.