It is enough to change one character in the file
C:\Program Files\CELSYS\CLIP STUDIO 1.5\CLIP STUDIO PAINT\resource\english\742DEA58-ED6B-4402-BC11-20DFC6D08040
31 to 30 and install the English version.
30 00 00 00 22 68 74 74 70 3a 2f 2f 77 77 77 2e
Wow! That actually works :o
Just quickly tested it with
Notepad++ and added the Hex-Editor plugin to it (Plugins > Plugins Admin > Hex-Editor) and did a search for
31 00 00 00 22 68 74 74 70 3a 2f 2f 77 77 77 2e
and changed 31 to 30

Edit: CLIP now seems to be using my EN license from the server instead of my JP.
Can anyone else confirm if this HEX editing works for them?