This behavior is by design. Language packs are not upgraded as part of the operating system upgrade. This issue occurs because Windows 8 and Windows 8.1 each have version-specific language packs. When you upgrade Windows 8 to Windows 8.1, the operating system reverts to its base language version. After the upgrade is finished, you have to reinstall any language packs that you require.
The instructions for installing language packs can be found in the following Microsoft Knowledge Base article: https://support.microsoft.com/en-us/kb/2...
It is also possible that the Advanced language options are set to cause Windows to use a display language that is not yet installed after the upgrade. This causes a condition that prevents you from being able to change the display language or download language packs.
There are advanced language settings that may block the download of language packs. To revert these settings to their defaults values so that you can download language packs, follow these steps:
Open Control Panel. To do this, type Control Panel in the Search box, and then tap or click Control Panel in the search results list.
Tap or click Clock, Language and Region. (If you are viewing Control Panel in icon display, select Language.)
Tap or click Language.
Tap or click Advanced settings.
Examine the Override for Windows display language and Override for default input method lists. Make sure that the Use language list (recommended) option is selected for both lists. Then, tap or click Save.
After you save the settings, you are returned to the standard language settings. In the list of previously installed languages, click Options next to the language you want to install.
Click the download link to install the language pack
Note After the installation is finished, you are prompted to restart the computer.