You can only install a language pack at install time in VS 2017 RC. To install RC with a different language:
- Open the Visual Studio Installer.
- Find an addition under “Available” and click Install
- Click on the “Language packs” tab and select a language
You can have multiple instances of VS 2017 side by side so this shouldn’t interfere with your other installation.
Disclosure: I work on Visual Studio at Microsoft.