Output discrepancy when building Translator

[skrivver99]

Hi, can someone please help me see what is wrong with the following code?

For example, I want the input "ceiling" to produce "силинг".

def translate(phrase):
    translation = ""
    for letter in phrase:
        if letter in "Aa":
            translation = translation + "a"
            # Capitals
        elif letter.lower() in "b":
            if letter.isupper():
                translation = translation + "Б"
            else:
                translation = translation + "б"
        elif letter in "Cc" and translation[+2:] == "ei":
            translation = translation[:+2] + "си"
        elif letter in "Ll":
            translation = translation + "л"
        #elif letter in "Cc":
            #translation = translation + "к"
        #elif letter in 'Ii' and translation[-2:] == "kе":
            #translation = translation[:-2] + "cи"
        elif letter in "Ii":
            translation = translation + "и"
        elif letter in "Ee":
            translation = translation + "е"
        elif letter is "cei":
            translation = translation + "си"
        elif letter in "Ff":
            translation = translation + "ф"
        elif letter in "Dd":
            translation = translation + "д"
        elif letter in "Gg":
            translation = translation + "г"
        else:
            translation = translation + letter
            '''
This list
Needs to be completed
'''
    return translation

print(translate(input("Enter word: ")))