To export your WordPress Site (even multi language based on qTranslate) into a flat file structure compatible with Grav start with the WordPress Plugin "WP2Grav", which you can install into your WordPress system:
Go to your WordPress Admin > Plugins > Search for "Wp2Grav"
Download on https://wordpress.org/plugins/wp2grav/
Clone the Github Repository https://github.com/SiteBeez/wordpress-wp2grav-markdown-exporter
Please read the Plugin Documentation to understand how the export works and how you can tweak it to work with your specific WordPress Installation.