Sample Gruntfile.js for RTL support using package `grunt-rtlcss` from https://www.npmjs.com/package/grunt-rtlcss

Gruntfile.js

module.exports = function( grunt ) {

	'use strict';

	// Project configuration
	grunt.initConfig( {

        rtlcss: {
            options: {
                // rtlcss options
                config: {
                    preserveComments: true,
                    greedy: true
                },
                // generate source maps
                map: false
            },
            dist: {
                files: [{
                    expand: true,
                    cwd: "assets/css/",
                    src: [
                        '*.css',
                        '!*-rtl.css',
                    ],
                    dest: "assets/css/",
                    ext: '-rtl.css'
                }]
            }
        }

	} );

	// Load task.
    grunt.loadNpmTasks( 'grunt-rtlcss' );

    // Register task.
    grunt.registerTask('rtl', ['rtlcss']);

};