@mehdi89, I feel your pain! I don’t have a solution, but I want to suggest a workaround. It will require your employees to install VirtualBox and Vagrant on their machines, and to use Git or other distributed version control to sync local workspace with your C9 workspace. If this is feasible, read on.
Vagrant uses a Vagrantfile to automatically create a virtual machine, and provision it for a specific purpose. This repository has a Vagrantfile that creates a lightweight Ubuntu VM and provisions it for running Cloud9IDE locally. It runs scripts to install the IDE and dependencies. The first time it needs to download and install everything in the VM, but after that it does not need Internet connection. It creates a local private network on your computer, so you just point your browser to this network’s IP to access the IDE. It creates a directory
workspace/ on local machine and maps it to the IDE’s
workspace/ on the VM. When Internet is available, your employees can push their work to a remote repository, log into C9-hosted workspace, then pull changes from that repository. Good Git/Mercurial habits will be necessary.
The repository has straightforward directions, but if you need help, just open an issue at GitHub and document the problem, or direct message me from here.