import re


def clean_sentence(sentence):
    """
    Remove unreadable Unicode characters and non-alpha characters from the text.
    
    :param sentence: Sentence to clean.
    
    :return text: Cleaned sentence.
    """
    text = re.sub(r'[^a-zA-Z\']', ' ', sentence)

    # remove Unicode characters
    text = re.sub(r'[^\x00-\x7F]+', '', text)

    return text