Raise error if translating from/to same language

Currently, if you click "Translate" while the same from/to language is selected, the spinner spins endlessly. Instead we want clicking "Translate" to `alert` an error saying that you cannot translate to/from the same language.

We also want to remove the " Translate To language is the same as the Source language" plaintext that appears at the bottom of the modal. Since the default language shown is often the same as the book.