I had problems with the woocommerce plugin.
MY SOLUTION:
wp-content\plugins\woocommerce\includes\class-woocommerce.php
in file around line 650 in the.
load_textdomain( 'woocommerce', WP_LANG_DIR . '/woocommerce/woocommerce-' . $locale . '.mo' );
I switched now to
load_textdomain( 'woocommerce', WP_LANG_DIR . '/plugins/woocommerce-' . $locale . '.mo' );
And delete this:
load_plugin_textdomain( ‘woocommerce’, false, plugin_basename( dirname( WC_PLUGIN_FILE ) ) . ‘/i18n/languages’ );