Replaces a .mo-file based on a text domain. In this case from within a theme. Two different methods. One replacing the plugin text domain all together, and one that just switches the .mo-file.

load_textdomain.php

add_action( 'after_setup_theme', 'myplugin_load_textdomain' );

function myplugin_load_textdomain() {
	
  load_plugin_textdomain( 'my-plugin', false, get_template_directory() . '/my-plugin-lang-dir-in-theme/' ); 
}

replace_mo.php

add_filter( 'load_textdomain_mofile', 'myplugin_new_mo', 20, 2 );

function myplugin_new_mo($mo, $domain) {


	if( $domain === 'my-plugin-textdomain' ) {

		$dir = get_template_directory() . '/my-plugin-lang-dir-in-theme/';

		$mo = $dir . 'coolfile.mo';

	}

	return $mo;

}