Translating from English to Entean
Use Dictionaries

We are basically just mapping from one letter to the next. The most naturally way to do that is to actually use a map:

to_entean = {...}


The easiest way to build up the dictionary is to use zip():

to_entean = {eng: ent
    for eng, ent in zip(string.ascii_uppercase, 'AZYXEWVTISRLPNOMQKJHUGFDCB')
    }


Now to_entean['A'] == 'A' and to_entean['B'] == 'Z', etc. This simplifies your get_letter function to:

def get_letter(letter):
    ent = to_entean.get(letter.upper(), letter)
    if not letter.isupper():
        ent = ent.lower()
    return ent


String Concatentation

String concatenation is slow. Also it reads terribly. What we want to do is call get_letter for every letter in word and then join them all together. That's what join() is for:

ent_word = ''.join(get_letter(letter) for letter in word)


Why split?

Since get_letter() handles spaces already, we don't have to split into words. Just transform the whole thing:

print(''.join(get_letter(letter) for letter in usr_txt))


Which, if you prefer, can also be spelled:

print(''.join(map(get_letter, usr_txt)))


Typo

You have a typo here:

usr_txt = input("Enter Englsih text:")
                       ^^^^^^^