Updated version (accounts for more than a digit, e.g 30):

def decode(word, index=0):
    word_set = []
    while index < len(word):
        char = word[index]
        if char == ']':
            break
        elif char.isdigit():
            count = 1
            while char.isdigit():
                char = word[index + count]
                count += 1
            multiplier = word[index:index+(count - 1)]
            new_word_set, index = decode(word, index + count)
            word_set = word_set + int(multiplier) * new_word_set
        else:
            word_set.append(char)
        index += 1

    return word_set, index

def decode_word(word):
    return ''.join(decode(word)[0])