Apologies, I'm not exactly sure what you mean - but if you've put the key in a lang file it would work for any level that the key is put in. If you mean making a key from one level opening a door in another, a callback in the level with the door should suffice
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The callback for displaying a message is SetMessage("Category", "Entry", X.0f)
X being the amount of time you want the message to be on the screen.
Edit: Also in the future you should probably post questions like this in the development support section, just a tip