A script for removing lines which are in wrong language from nmt datasets
Signed-off-by: PeganovAnton <peganoff2@mail.ru>

WMT parallel corpus contains many "broken" examples in which source or target are in a wrong language. The fractions of examples in WMT20 for which fasttext [lid.176.bin](https://fasttext.cc/docs/en/language-identification.html) model predicted language which differs from required:
commoncrawl: 7%
europarl: 0.85%
news_commentary: 4%
paracrawl: 0.5%
tilde: 25%
wikimatrix: 3%
wikititles: 25%
Some of bad examples are too short or contain only punctuation and many are in a wrong language.
The script goes through WMT 20 parallel corpus in 10 min if 20 CPU cores are used.
The script can also be used for filtering a monolingual corpus of arbitrary size.