A Language pack is a type of add-on for OpenOffice to get more languages integrated without to install more than one Office set. It contains only files to display, e.g., menus and dialogs in a specific language incl. the spell checking libraries (since OOo 3.2). If available the help content is also translated.
Where to download
Language packs can be downloaded on this download website. It is not a stand-alone release, so it has to be installed over a stable OpenOffice release with the same version number on the same platform.
Apache_OpenOffice_4.1.3_Win_x86_langpack_en-US.exe provides the language resources for English-US and can be installed only over an existing Apache OpenOffice 4.1.3 on Windows.
If you cannot find a language pack for your preferred language and platform it is possible that it is not yet available. Then you can download a Release Candidate. However, please note that these builds are not finally tested and therefore not yet released.
How to change the language
After the installation the language can be changed via the menu "Tools - Options - Language Settings - Languages - User Interface".