filter.py

#!/usr/bin/env python

def slicing(lang_idx, lang_cnt, lines):
    '''Using islice; short, but requires knowledge of total line count'''
    from itertools import islice

    return islice(lines, lang_idx, 12, lang_cnt)

def counting(lang_idx, lang_cnt, lines):
    '''Using count in conjunction with dropwhile; on and on and on and on...'''
    from itertools import count, dropwhile

    f = enumerate(lines)

    for c in count(lang_idx, lang_cnt):
        try:
            yield dropwhile(lambda e: e[0] < c, f).next()[1]
        except StopIteration:
            break

def rotate(l, n):
    return l[-n:] + l[:-n]

def compressing(lang_idx, lang_cnt, lines):
    '''Using compress in conjunction with cycle; also works on infinite iters'''
    from itertools import compress, cycle

    selector = rotate([True] + [False] * (lang_cnt - 1), lang_idx)
    return compress(lines, cycle(selector))

def main(strategy):
    print strategy.__doc__

    lang_cnt = 4

    for i in range(0, lang_cnt):
        print "Language", i

        for l in strategy(i, lang_cnt, open('input')):
            print l.strip()

        print ''


if __name__ == '__main__':
    main(slicing)
    main(counting)
    main(compressing)

filter.sh

#!/bin/bash -e

#set -o xtrace

for i in {1..4}; do echo "Language $i"; sed -nre "${i}~4 p" input; echo ''; done

input

English
Spanish
German
French
English
Spanish
German
French
English
Spanish
German
French

Cooked up in response to https://groups.google.com/d/msg/mumpy/jXGcvMkLWIk/2apgkf48EwAJ

Like I said in the forum, the batteries are included 🔋🔋 - ⚡ Power Up! ⚡