(from a response in forum item "Translation.xml scope in Designs" : http://developer.dynamicweb.com/Default.aspx?ID=2&CategoryID=30&ThreadID=37207)
It is not possible to auto create keys, as Nicolai stated.
Isn't that a nice to have feature? (perhaps configurable with another checkbox in the back-end, so it can be disabled on production sites)
For developers that use Dynamicweb it's impossible to keep track of all the Translate tags you've added in all the different templates.
I'd suggest, when a key isn't available during rendering, add it to the designs (global) xml with the default value for all current languages.
Almost all the adds will be happening during the development fase, because you view your own progress ...
We often have (during development) our clients asking for a list of to be translated keys.
But we fail to deliver each time, because we don't have this (complete) list on hand.