You need to make a copy of the file and call it "Shop.english-utf8.php". So, Shop.english-utf8.php should have the same contents as Shop.english.php (don't delete Shop.english.php).
Before you copy it, make sure the Shop.english.php file actually has stuff in it. If it doesn't, reupload it from the SMFShop package.
Oh, and I got the file name slightly wrong - It's "Shop.english-utf8.php" (utf8 is lowercase)