Do i need to restart the service for the language files to be used after updating?

Yes, after making any edits to the language file, you must restart the Cerberus FTP Server in the Window Service Manager. 

You can restart the Cerberus service by going to the Services control panel in Windows (go to the Control Panel and select Administrative Tools), selecting the service named Cerberus FTP Server, and then selecting the restart option for the service.