You can specify the date representation for each of your clients at "Client Profile > Locale Format". There, you can switch to Custom Values and change your long date format to DD.MM.YY so it is displayed as a short date format (Picture 1). Then you will have no system language words in your invoices. If you enter MMMM instead of MM - then your month will be shown using your default Windows language again.

You can also change your default Windows date representation in the same way, to apply these changes to all clients by default, by following the steps from the following article:

http://windows.microsoft.com/en-us/windows/change-dates-times-currency-measurements-display#1TC=windows-7