Log in as client administrator (in GardenWorld e.g.).
Go to Menu > Application Dictionary
There is one entry named "Window Customization".
The help text says:
"Define Window Customization for Role/User
The customization values defined here overwrite/replace the default system definition if defined."
The related functionality is not complete and shall be enhanced.
In the tests found this bug in the log:
-----------> MUserDefTab.get_Value: Column not found - ReadOnlyLogic 
But not sure where it was raised.
I don't get clear why the language is mandatory - it requires that you configure the customization per each language? [Note: Not reviewed thoroughly]
If you want we can check these design details on the iDempiere meeting tomorrow (maybe is fine just that I haven't reviewed deep).
thanks for testing.
I have added the missing column.
Concerning language, yes, as you can alter name and description you have to choose a language for that. But as you said this may mena to repeat setting for each language. Maybe we can also use '*' here to mean 'any language'.
We can discuss it in the meeting today.
Here are the current patch and the migration scripts