Although it would be very difficult to screw something up while following a guide, it is still possible to cause some sh*t to go wrong by recklessly changing registry entries. Having said that you'll do this at your own risk and, neither I nor Valve nor the friend who sent you here, can or will be held responsible for anything that goes wrong.

Step 1: Press the windows key.
Step 2: Type 'regedit'
Step 3: Press enter.

Easy right? You got the registry editor open.

Step 4: On the left bar find the section HKEY_CURRENT_USER and expand it.
Step 5: Find the folder named SOFTWARE and, also expand it.
Step 6: Find the folder named Joey Drew Studios and, you guessed it, expand it.
Step 7: Find the folder named Bendy and the Ink Machine and click on it.

Now you got the entries for the game on the right side.

Step 8: On the right side, find the registry key named LANGUAGE_h2417763223 and double click on it. At this point its value should be 00.
Step 9: Edit the hex value to 65 6E 00 (basically "en." in hex) (see screenshot).
Step 10: Save it.

You're done. Enjoy.

Note: I'm certain that if you know the hex values for other languages you could change to other than english. However I didn't try it.