If you want to upgrade Windows (ex. 8.1 th-th to 10 en-us) Then, Windows Setup won't allow you to upgrade by keeping all programs and settings. Unless you find Windows 10 th-th iso. There is another alternative method though. Change Window base language to en-us. Run this command to check the current Default system UI Language: DISM /online /get-intl Then to change base language, you need to boot into Windows Recovery Command Prompt. DISM /image:C:\ /set-uilang:en-US Change Windows base lang to en-US. (It dones instantaneously) Now when you boot windows, you'll notice that login screen language has change coressponding to your base language. Now you can upgrade to Windows 10 using en-US iso.
I believe there is no point in doing this. There was user who upgraded his Italian Windows 8.1 copy to Windows 10 en-us (files intact) by changing the InstallLanguage dword in his 8.1 copy: 0409 is en-us