[Doitsu]

@Vroni:

The following minimal code should get you started:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import sys
from sigil_bs4 import BeautifulSoup

def run(bk):
    metadata_soup = BeautifulSoup(bk.getmetadataxml(), 'lxml')
    dc_language = metadata_soup.find('dc:language')
    if not dc_language:
        dc_language = metadata_soup.new_tag('dc:language')
        metadata_soup.metadata.append(dc_language)
    dc_language.string = 'en-US'
    new_metadata = str(metadata_soup.prettyprint_xhtml())
    bk.setmetadataxml(new_metadata)
    print('Done')
    return 0

def main():
    print('I reached main when I should not have\n')
    return -1

if __name__ == "__main__":
    sys.exit(main())

It'll change the language code to en-US or add a new en-US language metadata entry.