If you try to add an additional language pack in Windows 8 RTM, you might see the following message in the control panel: "A language pack isn't available". In the same place, Windows 7 would show a download link. So I'm guessing this is a temporary condition, as Windows 8 is not slated for general availability until October 26.
Still, there is a way to install language packs right now, as long as you have access to MSDN Subscriber Downloads. And if you are already using Windows 8 RTM, there's a pretty good chance that you do.
We start with the steps to install a language pack in Windows 8 that you would normally follow
1. Open the Control Panel.
2. Under Clock, Language and Region, click Add a language.
3. Click Options next to the language for which you want to add a language pack.
4. It says "A language pack isn't available". But we can download language packs from MSDN Subscriber Downloads instead.
5. In MSDN Subscriber Downloads, navigate to Windows 8. Browse the list of downloads until you find Windows 8 Language Pack (x64) - DVD (Multiple Languages), or the x86 version respectively. Download it.
6. When the file is downloaded, right click it in File Explorer, and select Mount. Now you can access it as a virtual CD drive.
7. Press Windows Key + R to open the Run dialog.
8. Type lpksetup and press Enter.
9. Click Browse, and find the language file that you want to install from the virtual CD. Then click Next and follow the instructions.
10. When the installation is done, you can go back to the Control Panel. It should now look something like this.
Now you are done.