docs: translations: use attribute to store current language

Akira Yokosawa reported [1] that the "translations" extension we added in
commit 7418ec5b ("docs: translations: add translations links when they
exist") broke the build on Sphinx versions v6.1.3 through 7.1.2 (possibly
others) with the following error:

    Exception occurred:
      File "/usr/lib/python3.12/site-packages/sphinx/util/nodes.py", line 624, in _copy_except__document
        newnode = self.__class__(rawsource=self.rawsource, **self.attributes)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    TypeError: LanguagesNode.__init__() missing 1 required positional argument: 'current_language'
    The full traceback has been saved in /tmp/sphinx-err-7xmwytuu.log, if you want to report the issue to the developers.

Solve this problem by making 'current_language' a true element attribute
of the LanguagesNode element, which is probably the more correct way to do
it anyway.

Tested on Sphinx 2.x, 3.x, 6.x, and 7.x.

[1]: https://lore.kernel.org/all/54a56c2e-a27c-45a0-b712-02a7bc7d2673@gmail.com/

Fixes: 7418ec5b

 ("docs: translations: add translations links when they exist")
Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Closes: https://lore.kernel.org/all/54a56c2e-a27c-45a0-b712-02a7bc7d2673@gmail.com/
Tested-by: Akira Yokosawa <akiyks@gmail.com>  # Sphinx 4.3.2, 5.3.0 and 6.2.1
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215064109.1193556-1-vegard.nossum@oracle.com