How to change system language everywhere?
The truth is that you cannot change an Italian version of Windows into an English version.
Microsoft offers very many language packs that will translate the main elements of the interface to English. And they work very well. (No bugs.) But you still have an Italian version of Windows, so it's not going to be completely English. If you want a truly English version of Windows, i.e., English in every way, then you have to purchase and install one of the native English versions of Windows.
In addition to being completely in the English language, the native English version of Windows identifies your computer as English to websites and to software, so you'll see the English version of websites and when you install software (and updates thereto) you'll be installing the English version. And there's more:
The native English version of Windows incorporates the styles and idioms of the English-speaking country whose version of Windows you are installing. There are three native English versions of Windows of which I am aware: United States, Australia and Canada. So, for example, if you were to install the English-US version of Windows you would see US expressions rather than English translations of Italian expressions.
If you decide to install a native English version of Windows, I recommend that you purchase a keyboard designed for that language. That way you'll avoid typing one thing and seeing a different thing on your monitor, which can sometimes happen when the languages are mixed.