Add missing translation

app/controllers/spree/admin/orders_controller_decorator.rb

@@ -79,7 +79,7 @@ Spree::Admin::OrdersController.class_eval do
     template = if Spree::Config.invoice_style2? then "spree/admin/orders/invoice2" else "spree/admin/orders/invoice" end
     pdf = render_to_string pdf: "invoice-#{@order.number}.pdf", template: template, formats: [:html], encoding: "UTF-8"
     Spree::OrderMailer.invoice_email(@order.id, pdf).deliver
-    flash[:success] = t(:invoice_email_sent)
+    flash[:success] = t('admin.orders.invoice_email_sent')
 
     respond_with(@order) { |format| format.html { redirect_to edit_admin_order_path(@order) } }
   end

config/locales/en.yml

@@ -378,6 +378,7 @@ en:
       index:
         capture: "Capture"
         ship: "Ship"
+      invoice_email_sent: 'Invoice sent'
       bulk_management:
         tip: "Use this page to alter product quantities across multiple orders. Products may also be removed from orders entirely, if required."
         shared: "Shared Resource?"