I don't think that any of them will translate phrases for you. The Sony has integrated dictionary support with five languages supported (French, Italian, German, Spanish, and Dutch) for translation to and from English. There are also two English language dictionaries that provide definitions. Double tap on a word and a little window pops up at the bottom of the page with the definition. Tap anyplace on the page outside the window and the window goes away. Tap inside the window and you get the full dictionary page.
Personally, I think that the Kindle and Sony both have better software than the Kobo. For the Kobo and the Sony, you can buy books from anywhere except B&N, Apple, and Amazon. www.inkmesh.com
is a good way to see where specific books are offered. Before Agency pricing, it was also a good place to find bargains. As CRussell said, it's quick and easy to remove DRM and then use Calibre to convert to the format of your choice. If you're willing to do that, then you can buy from anyplace but Apple.
The page turns, responsive screen, and screen quality is virtually identical for all three readers. All e-ink readers have a flash when you turn the page, but you don't notice that any more than you notice the pause as you turn the page in a paper book.