Thank you for the solution!
(was searching in vain for years)
It worked instantaneously!
I can take "english - europe" for the region prefz no-problem, because it is the same as my country altogether indeed!
Thankx again to have posted this!
PS: i found the exact registry key that BSplayer utilise to auto-pick the ST:
Ordinateur\HKEY_CURRENT_USER\Control Panel\International\LocaleName
(the value should be "Eng" [without ""] to auto-pick Eng ST so)
You can create/export the registry key and put it on your desktop to activate any language (it works instantanneously with BSplayer) at will... (if like me you have some problems with changing the region format, like some characters with accent not displaying correctly on some programs etc...)