A single dont_hack() could replace the three-liner if(IN_serendipity){}. This is some PhpNuke-era inline code execution prevention boilerplate. IMO this has no place in properly architected apps, but I realize some plugins might indeed still have some autoexecuting code.
By switching to a function like "dont_hack()" you basically get a fatal error if that wasn't defined before. So in essence a simple function call replaces the manual constant check. Also, I'd use less pity wording for the die() error message, or rather a different name for this replacement function. Name suggestions!
The $probelang code can also be centralized easily. These 7 lines in every plugin could be supplanted by a simple include_lang(__FILE__). Additionally this prepares Serendipity for a transition off oldschool translation constants. (I have something gettext-esque in mind.)
Both changes are simple and non-intrusive enough, but I'd volunteer a patch or branch, of course. <- temporary developer self-invite