add_action ( 'flexible_invoices_pre_generate_pdf', 'my_flexible_invoices_pre_generate_pdf' );

function my_flexible_invoices_pre_generate_pdf( $invoice ) {
    global $locale; $locale = 'en_US';
    unload_textdomain( 'flexible-invoices-woocommerce' );
    load_plugin_textdomain( 'flexible-invoices-woocommerce', false, 'flexible-invoices-woocommerce/lang/' );
    unload_textdomain( 'flexible-invoices' );
    load_plugin_textdomain( 'flexible-invoices', false, 'flexible-invoices/lang/' );
    unload_textdomain( 'woocommerce' );
    load_plugin_textdomain( 'woocommerce', false, 'woocommerce/i18n/languages/' );
}