I think Memag is correct, it doesn't use "C:\Program Files\CELSYS\CLIP STUDIO 1.5 EN" any more.
My system only has the Japanese version installed into the normal "C:\Program Files\CELSYS\CLIP STUDIO 1.5" folder.. but its the same folder when I installed the English version - whatever serial you use is the language which gets installed.
So from what you're saying... it looks like you can't have both languages installed on the same machine using the installer?
Try this, install the English EX serial version, run it, close it and then make a copy of the folder and rename it to "CLIP STUDIO 1.5 EN".
Now install the Japanese Pro serial version and it should update the original "CLIP STUDIO 1.5" folder to Japanese.
Since both applications will reference /My Documents/ for your settings & materials, you should now have both versions installed. Might be a pain to update them both all the time this way though.
We should email CELSYS and ask them if it's possible to have both versions at the same time!
If the JP/EN serials could install either language then the program would just need a simple switch to pick the language (like /L1033). I wonder if CELSYS already have a secret function like that already built in...
EDIT:
If I was you, I'd actually delete your old "CLIP STUDIO 1.5 EN" (this folder doesn't get updated it seems) and install your English EX serial version