This error can occur when attempting to upgrade from Office 2007 to Office 2010 and you are installing Office 2010 from the MSOCache.

To reproduce this behavior: 

Install Office 2007.
Then use the following procedure to deploy the local installation source before you plan to install Office 2010. (Precache)

To deploy the local installation source separately

1. On the network installation point, open the Config.xml file in a text editor, such as Notepad. For information about the Config.xml file, see Config.xml file in Office 2010.

By default, Config.xml is located in the core product folder for the Office product that you are installing. For example, if you install Microsoft Office Professional Plus 2010, open the Config.xml file in the ProPlus.WW folder.

2. Find the LIS element (<LIS>); remove the comment marks in the line by deleting the opening <!-- and closing --> tags.

3. Set the <CACHEACTION> attribute to "CacheOnly".

The line in Config.xml should look as shown in the following example.

<LIS CACHEACTION="CacheOnly" />

4. Save the Config.xml file.

5. Run Setup.exe on users' computers; on the Setup command line, specify the path of the modified Config.xml file.

You must use a fully qualified path. For example: “\\server\share\Office2010\setup.exe /config \\server\share\Office2010\ProPlus.WW\config.xml”

Where Office2010 is the root of the network installation point.

Then attempt to install the Office 2010 upgrade following the same steps as is recommended if you were installing Office 2007 from a precache deployment.

Precache the local installation source for the 2007 Office system
http://technet.microsoft.com/en-us/library/cc179231(office.12).aspx

"C:\MSOCache\All Users\{90140000-0011-0000-0000-0000000FF1CE}-C\setup.exe" /config \\server\share\Office2010\ProPlus.WW\Config.xml

Results-
Error: “The Language of this installation package is not supported by your system”

The reason that this error occurs is because when Office 2007 is installed and it has it’s MSOCache present, and then you have the cache for Office 2010 present, the setup.exe from the cache folder ({90140000-0011-0000-0000-0000000FF1CE}-C) queries the MSOCache for all of the products that it could install. It incorrectly identifies office 2007 as an available product to install, believes the MSOCache is a “combined source”, and crashes.

Workaround-
Rather than starting the Office 2010 installation from the setup.exe that resides in the MSOCache folder, start the Office 2010 install from the original network share that you had used to precache the installation.

*note*
You will need to ensure that you are not using the same config.xml that you had previously used to perform the cacheonly action, (or remove the CacheOnly line) or it will simply repeat the cacheonly action rather then perform the install.

This method will not copy additional data to the machine as it will detect that the MSOCache already exists.