def koreni(lists):
    out = []
    for list in lists:
        first_letter = list[0][0].lower()
        correct = True
        for s in list:
            if len(s) > 0 and s[0].lower() == first_letter:
                continue
            correct = False
        if correct: out.append(list)
    return out

print(koreni([['bavto', 'Bcbeceda'],['avto', 'abeceda', ''],['td', 'ts']]))