Loading theme and plugin translations in WordPress - https://ulrich.pogson.ch/load-theme-plugin-translations

functions.php

<?php
function theme_name_setup(){

	$domain = 'theme-name';

	// wp-content/languages/theme-name/de_DE.mo
	load_theme_textdomain( $domain, trailingslashit( WP_LANG_DIR ) . $domain );
	// wp-content/themes/child-theme-name/languages/de_DE.mo
	load_theme_textdomain( $domain, get_stylesheet_directory() . '/languages' );
	// wp-content/themes/theme-name/languages/de_DE.mo
	load_theme_textdomain( $domain, get_template_directory() . '/languages' );

}
add_action( 'after_setup_theme', 'theme_name_setup' );

plugin-name.php

<?php
function plugin_name_load_plugin_textdomain() {

	$domain = 'plugin-name';
	$locale = apply_filters( 'plugin_locale', get_locale(), $domain );

	// wp-content/languages/plugin-name/plugin-name-de_DE.mo
	load_textdomain( $domain, trailingslashit( WP_LANG_DIR ) . $domain . '/' . $domain . '-' . $locale . '.mo' );
	// wp-content/plugins/plugin-name/languages/plugin-name-de_DE.mo
	load_plugin_textdomain( $domain, FALSE, basename( dirname( __FILE__ ) ) . '/languages/' );

}
add_action( 'init', 'plugin_name_load_plugin_textdomain' );