[docs] set language = 'en' in Sphinx config

[jameslamb]

Noticed on #5320 that the check-docs CI job has started failing with the following error.

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
make: *** [Makefile:20: html] Error 2

Per sphinx-doc/sphinx#10481, it looks like Sphinx no longer accepts language = None in configuration. (found that link from rapidsai/cuml#4781, thanks @galipremsagar!).

This PR proposes explicitly setting language = 'en' in the project's Sphinx configuration.

[jameslamb]

Thanks for the quick review @jmoralez !

[github-actions]

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