You can change the language to English with these steps:
- Start calibre
- Type CTRL P to open calibre preferences
- Click the "Paintbrush" icon (see image)

- Click the language box (4th down, see image)

- Choose English. You will see the word "English" no matter which language is being used.
- Press the Apply button, bottom right corner of the dialog box.
- Press the Close button, bottom right corner of the dialog box.
- Quit calibre (CTRL Q)
Alternatively, if you just installed calibre and have made no changes to your preferences then you can delete the calibre preferences folder C:\Users\[yourname]\AppData\Roaming\calibre then restart calibre. You will see the calibre Welcome Wizard again.