You may not omit the “Exec=”!
But I tried myself now and it is true that GNOME doesn’t like those envvars in the Exec line and doesn’t show the entry if they are there (and double-clicking on it in nautilus only gives an error that gnucash couldn’t be started).
I have a better idea though:
Create a script named “gnucash” that runs it with those envvars set, and put it to /usr/local/bin/. As /usr/local/bin/ is in the path before /usr/bin/, your script will get called instead of gnucash. This will also work for the command line… 
The script should probably look like this:
#!/bin/sh
LANGUAGE=de_DE.UTF-8 LANG=de_DE.UTF-8 /usr/bin/gnucash $@
($@ will append all parameters that are passed; and don’t forget that /usr/bin or the shell will be in an endless recursive loop calling the script over and over again which will make your system unuseable sooner or later…
)
And don’t forget to make it executable:
sudo chmod +x /usr/local/bin/gnucash
Or, if you want to restrict that to one user, put that script somewhere into your home directory and create a ~/.local/share/applications/gnucash.desktop that calls this script instead of gnucash.
You can also combine this to have two different menu entries, one normal and one forced to german. Just name that script differently, and create a new .desktop file (in /usr/share/applications/ or ~/.local/share/applications/) with a different name that calls the script instead of gnucash…
You are right about the renaming. Under Gnome, the icon subscript is GnuCash, but the actual (not shown) file name is gnucash.desktop. Nautilus only shows the Properties ‘Name’ field as the icon subscript, which is not the same as file name which seems to remain almost totally inaccessible.
Well, I’m not totally against the idea to present the files that way, but there should at least be an option to show the plain files as they are with their filename. (maybe there is, but I haven’t found it yet?)
It is a file manager after all, or at least it should be…
I was getting used to Gnome, but this behaviour is making me nervous. I could never find a KDE theme that I liked. How can I change to KDE?
Just install it and you should be able to choose it at the login screen. You can have several desktops installed side-by-side without problems.
Probably the easiest way to install it is via YaST->Software Management, View->Patterns.
There is a GTK+ style for KDE (installed by default) which should allow you to use any GTK (2 I think) theme as well.