[all][sphinx] "rST localisation for language "foo" not found" error

We've seen issues in some projects, whereby attempting to build translations as part of the 'openstack-tox-docs' zuul job [1] can result in the following warning:

Warning, treated as error: ../doc/source/contributor/contributing.rst::rST localisation for language "id" not found.

Based on [2], this issue appears to have been introduced by a recent version bump of docutils, which was in turn introduced by the introduction of Sphinx 4.x.

We don't have a resolution for this bug yet but we'll update when we do. Hopefully this email will improve the "Google'ability" of this bug.

Stephen

[1] https://opendev.org/openstack/openstackdocstheme/src/commit/08461c5311aa6920... [2] https://lists.freebsd.org/pipermail/freebsd-ports/2021-April/120848.html