I got an answer from a Microsoft member:
With Windows Vista and Windows Server 2008, the binaries (exe, dll,
etc.) are language neutral - that means they do not have any resources
(such as strings) that require translation/localization to specific
languages. All of the resources (including English) are stored in
separate files that rarely require updates. For most Vista updates
(MSUs), only the core binaries that don't have any language dependency
need to be updated. This means the same language neutral package can
be installed on any language version of Vista. So the file itself is
Language Neutral. The XML files specify the applicability for the
update, meaning that such a package "applies to" all languages.
If we do need to update the resources, then it requires a package with
all or some subset of languages.
Vista SP1 is an example where we definitely had to update resources as
well as code. We released two versions: a "5 language" covering
English/Japanese/German/Spanish/French and a "36 language" version
covering all the primary localized languages for Vista. For SP1,
there are sub-packages inside the MSU that have the localized
resources. Therefore, the 36 language version also applies to "all"
language editions, but carries the appropriate resources for whatever
language it covers. There were two reasons we also released the 5
language version: (1) the localization and testing for these languages
was completed about a month earlier than the last of the 36 languages
so we could release it sooner, and (2) the package size without all
the other 31 languages was smaller which is important for some
corporate deployment customer scenarios.