I need help! D:

[Datguy5]

There is no error message; the map loads just fine. But SetMessage (which, according to the wiki, should display a message on the screen) won't work when it's supposed to at the end of the timer, the screen just remains blank, and when I use a CustomSubItemTypeName on a key in the level editor it only gets rid of the default name and description instead of showing the custom name and description.

And from what a tutorial told me, the name of the key and the CustomSubItemTypeName can be completely different, but you need to include the CustomSubItemTypeName in the LANG file. Is that wrong?

Well, your line is probably working just fine.
SetMessage("DA01Map01", "AdrianShock", 0);
should display AdrianShock, in the DA01Map01 category, for 0 seconds.
Try "1.0f" instead. (Edit: Oh, didn't read the part about 0 setting it to text length. Try "0.0f".)

...and as for the key, that's correct: If a CustomSubItemTypeName of a key is "butler", then the entry names for that key should read "ItemName_butler" and "ItemDesc_butler". (The name is of course case sensitive, so "ItemName_Butler" wouldn't work.)

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I'm at a loss with this. I'm not experienced enough. It could be that AngelScript has a limitation on the number of "else if" statements. It could be anything. You could test SetMessage outside the code and see if it works then.
Does your story find anything else in your lang file? Maybe the lang file is in a folder it shouldn't be in.

which folder does the extra_english.lang file have to be under?

It has to be in your custom story folder.Not in the maps.(The same as custom story settings.cfg)