Clicking the Download Language Packs button will open your web browser and bring you to the Autodesk Knowledge base website where you can download the language pack you want. Scroll down until you find the language you want to install and click the hyperlink to download the file.
There will be several languages to download and the list of available languages will vary depending on your specific region. Here is a partial list from my selection options and a glimpse as to what the website will look like.
Click on the hyperlink of the language you want to download. Make sure to pick the system type that you need. There are 32 bit and 64 bit versions. Match your computer’s system type. This downloads a self-extracting .exe file that will install the language pack similarly to how AutoCAD is installed.
Once the .exe file is downloaded run the file. This will extract the needed files to a folder on your computer and should start the install program. If it doesn't, browse to where the files were extracted to and run the setup.exe file in the folder.
Once the installer has started click the INSTALL button on the bottom right of the screen. The installer instructions will be in the language of the language pack, but otherwise it will exactly the same.