Unlike Window 7, to change the system language of Window 8.1, you won't have to download the Vistalizator software or the MUI (Multilingual User Interface Pack) pack.
Press the Windows key + C, and click on Settings.
Click on "Change PC settings".
Click on "Time and Language".
Go to Region and language and click on the + button to add a new language.
Select the language of your choice and follow the onscreen instructions:
Open the Control Panel > Language.
Click on Add Language.
Select the language (or a variant of the language).
In our example: Deutsch > Add and confirm.
The selected language appears.
Note that: Your PC must be connected to the internet, to download the language packs.
To the right of the language chosen to click on Options.
Click on "Download and install the language pack"
The pack will be downloaded and installed.
Note that: Upon installing a new language, a new keyboard will also be installed (with letters, symbols specific to the language). You can switch between input methods by pressing the Win + Space key combo or by clicking on the tray icon: