docs: translations: use attribute to store current language

Akira Yokosawa reported [1] that the "translations" extension we added in
commit 7418ec5 ("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: 7418ec5 ("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

Documentation/sphinx/translations.py

@@ -29,10 +29,7 @@
 }
 
 class LanguagesNode(nodes.Element):
-    def __init__(self, current_language, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
-        self.current_language = current_language
+    pass
 
 class TranslationsTransform(Transform):
     default_priority = 900
@@ -49,7 +46,8 @@ def apply(self):
             # normalize docname to be the untranslated one
             docname = os.path.join(*components[2:])
 
-        new_nodes = LanguagesNode(all_languages[this_lang_code])
+        new_nodes = LanguagesNode()
+        new_nodes['current_language'] = all_languages[this_lang_code]
 
         for lang_code, lang_name in all_languages.items():
             if lang_code == this_lang_code:
@@ -84,7 +82,7 @@ def process_languages(app, doctree, docname):
 
         html_content = app.builder.templates.render('translations.html',
             context={
-                'current_language': node.current_language,
+                'current_language': node['current_language'],
                 'languages': languages,
             })