doc: set language for Sphinx

When I upgraded Sphinx to 5.3.0 from Debian bookworm, I got the error:

Warning, treated as error:
Invalid configuration value found: 'language = None'.
Update your configuration to a valid language code.
Falling back to 'en' (English).

This fixes it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>