Include 'LANGUAGE' in the default metadata.

While building the '.po' with Babel (pybabel) it complains about this field being empty:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".venv/lib/python3.10/site-packages/babel/messages/frontend.py", line 929, in main
    return CommandLineInterface().run(sys.argv)
  File ".venv/lib/python3.10/site-packages/babel/messages/frontend.py", line 853, in run
    return cmdinst.run()
  File ".venv/lib/python3.10/site-packages/babel/messages/frontend.py", line 622, in run
    catalog = read_po(infile, locale=self.locale)
  File ".venv/lib/python3.10/site-packages/babel/messages/pofile.py", line 381, in read_po
    parser.parse(fileobj)
  File ".venv/lib/python3.10/site-packages/babel/messages/pofile.py", line 308, in parse
    self._process_comment(line)
  File ".venv/lib/python3.10/site-packages/babel/messages/pofile.py", line 267, in _process_comment
    self._finish_current_message()
  File ".venv/lib/python3.10/site-packages/babel/messages/pofile.py", line 204, in _finish_current_message
    self._add_message()
  File ".venv/lib/python3.10/site-packages/babel/messages/pofile.py", line 198, in _add_message
    self.catalog[msgid] = message
  File ".venv/lib/python3.10/site-packages/babel/messages/catalog.py", line 628, in __setitem__
    self.mime_headers = _parse_header(message.string).items()
  File ".venv/lib/python3.10/site-packages/babel/messages/catalog.py", line 429, in _set_mime_headers
    self._set_locale(value)
  File ".venv/lib/python3.10/site-packages/babel/messages/catalog.py", line 317, in _set_locale
    self._locale = Locale.parse(locale)
  File ".venv/lib/python3.10/site-packages/babel/core.py", line 268, in parse
    parts = parse_locale(identifier, sep=sep)
  File ".venv/lib/python3.10/site-packages/babel/core.py", line 1094, in parse_locale
    raise ValueError('expected only letters, got %r' % lang)
ValueError: expected only letters, got ''
wichert / lingua

Include 'LANGUAGE' in the default metadata. #106
