How do I categorise non-english email using procmail and command line tools?

I am subscribed to a mail list where some of the messages are non-english which I cannot understand.

How do I filter the non-english messages to /dev/null using procmail and/or command line tools?

I use procmail to filter my email, so ideally any alternative tool would also require a procmail recipe.

I'd prefer not to have to train my own language models.

One way is to use the perl TextCat package from Gertjan van Noord.

The text_cat script outputs the most likely language for the mail. This recipe assumes text_cat has been installed under /usr/local/bin.

Here is a simple procmail recipe to call the text_cat script:

:0
* ^Subject.*Jobs.*Board
{
    LANG_=`/usr/local/bin/text_cat`

    :0
    * ! LANG ?? ^english$
    /dev/null

    :0
    jobs/
}

I've been running text_cat for a few years. There haven't been any non-english messages classified as english, that is, no false-positives. I've not been rigorous about checking for false-negatives.

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A second way, as mentioned by tripleee in a comment, is to use the language categorisation provided by spamassassin which also uses the text_cat script. Spamassassin will unwrap any MIME transfer encodings which the vanilla text_cat version above won't.

Here is an incompletely tested procmail recipe for filtering on the spamassassin X-Spam-Languages header:

:0
* ^Subject.*Jobs.*Board
{    
    # Delete non-english language emails using spamassassin header
    # Test for not X-Spam-Languages: en
    :0
    * !^X-Spam-Languages: en$
    foreign/

    # Save english language mails in folder
    :0
    jobs/
}

Warning: spamassassin will occasionally provide multiple language categorisations like so:

X-Spam-Languages: en da ro

which the above recipe does not account for.

Spamassassin Language Categorisation Configuration

Edit /etc/spamassassin/v310.pre and uncomment the following line:

loadplugin Mail::SpamAssassin::Plugin::TextCat

Configure the plugin in /etc/spamassassin/local.cf:

ok_languages en       # I understand english
inactive_languages '' # Enable all languages
add_header all Languages _LANGUAGES_
# score UNWANTED_LANGUAGE_BODY 5 # Increase score - not necessary and not recommended 

This recipe was incompletely tested with spamassassin version 3.4.2.

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To adapt these answers to excluding a different language would involve substituting the other language for english in the first case and substituting the other 2 character language code for en in the second case.