I stumbled on this because the German translation "Seitenvorlage" does not match
the English "Slide Design". Then realized that even in English the "Slide
Design" dialog is incorrectly named, as it deals with what is mostly done in the
new task pane in the "Master Pages" area. 

So, instead of just changing the title of the Slide Design dialog, and the menu
and toolbar entries that call it, to "Master Page" or something similar, I
suggest removing the separate "Slide Design" dialog and the associated "Load
Slide Design" dialog from Impress altogether. 

The "Slide Design" functionality is partly already possible in the new task pane
(e.g. exchanging the master for the selected slide can be done in the context
menu). The rest could be added.

The current "Load Slide Design" dialog is a legacy issue and has several defects: 
- The text boxes do not accept text although they look active, the "Categories"
listed are not all able to be loaded although the preview shows them and you can
click OK. 
- If you select a Template other than for a presentation, nothing gets loaded to
the Slide Design dialog. But no error occurs.

I recommend the following solution:

- Rename Format menu item, the context menu item (submenu at "Slide") and icon
on the toolbar
Slide Design ---> Master Pages

- Retarget focus of the menu item and toolbar to the task pane, "Master Pages" area
(for example, this is already the behavior when you choose Format - Slide Layout)

- Retain 2 features from current "Slide Design" dialog by transfering them to
context menu somewhere in Master Pages area:
1) Load additional master pages (currently achieved via "Load..." button in the
"Slide Design" dialog)
2) delete unused master pages (see current check box at bottom of "Slide Design"
dialog)

Please consider enhancing the usability in OOo/SO in this way. Thanks!