After the Installation, the first thing you need to do is to connect the recalbox to your TV with the HDMI cable.
To power on the recalbox, just plug the micro USB power cable in.
In the system selection screen, press START and choose > System settings > Language > Select the desired language then Close
Recalbox will then reboot, and the EmulationStation interface will be in the selected language.
In the system screen press START and choose > Quit > Shutdown System.
Wait for green/orange light on PI to stop flashing (to prevent any damage to the MSD card.)
Unplug the power cable.
Recalbox comes with a few homebrew games for several systems, it would be wise to try them out to see if everything is working as intended before going deeper.