I agree entirely that this latest version 1.7.3 is the worst ever version released and one can only assume that Wiedmann has created an XAMPP that you are supposed to use only the defaults with.
If you want to stray from the defaults then you should be as clued up as he seems to think he is so you can try and fathom out his sloppy narrow minded coding "do what I say you should do or accept the consequences" is the way he sees things.
Prior versions or at least up to 1.7.1, were controlled by a different developer with only some scripts supplied and edited by Wiedmann but since he took over the whole development all hell has broken loose - BTW, he seems to have done a runner.
Once you delete both xampp folders in the htdocs folders and all index.php and index.html files that are associated with redirecting to the Welcome pages then you can't access the Welcome page and so have no need to change or edit the language files as the language files only effect the Welcome splash pages.
As I said the security pages have there own language files in C:\xampp\security\htdocs folder, so no need to do anything there when you access http://localhost/security
On the Welcome page the only links other than the security page are demos etc. of which you should be familiar by now and could do without them.
Once cleared of the xampp folders in the htdocs folder you can proceed to place your own files and folders in there and all should be fine to move forward.
Fingers crossed and good luck.