def get_lang_start_ind(doc_wordlens, doc_names):
    """Takes in a Pandas DataFrame containing the word length histograms
    for ALL documents, doc_wordlens, and a dictionary that maps each column 
    name (keys) to a string that describes each document (values).  Returns
    a list of each unique language in the document description, doc_langs, 
    and the corresponding starting row index for each language, doc_lrsinds.
    """
    doc_langs = []
    doc_lrsinds = []
    for ci, cn in enumerate(doc_wordlens.columns):
        lang = doc_names[cn].split("_")[0]
        if lang not in doc_langs:
            doc_lrsinds.append(ci)
            doc_langs.append(lang)
    return (doc_langs, doc_lrsinds)