By default, Ninite installers install the applications in the current language of the user's operating system. This can be overridden with the /locale switch, for example (for French):
NiniteInstaller.exe /locale fr-FR
This downloads and installs the applications in the specified language. For some applications, the user interface language will still have to be changed manually in the application's settings after the installation is complete.
See here for a list of locale strings. Please specify both the language and country code, e.g. "en-US", not just "en".