Set Cheetah environment to "en_EN.UTF-8"

[termdew]

Our error messages defined in 'lib/exceptions.rb' assume that the commands executed by Cheetah are running in an English environment.
When running into an SSH error on a system with non-English error messages, the user only gets the backtrace.

Therefore it should be ensured that Cheetah runs with "LANG=en_EN.UTF-8".