I'm having a bit similar problem with a multilanguage site. This must be trivial, but as a newcomer I have spent hours to find out. To put it simply: How do you make this:
If I'm not wrong you mean that you want to use the same name for the URL alias: "customers" and use a different name in the menu: "clients" / "kunden"
Never tried it, but a option is to edit the classes and add a specific attribute for the URL alias (non-translable) and configure the "URL alias name pattern" attribute with this new attribute.
The menu template will create the menu with the content object names: "clients" & "kunden", but the URL alias will be generated with "URL alias name pattern".