The main reason not to delete the dictionaries is if you ever read a book in a language other than English. For example, if the book is flagged as being in French (via a setting in its meta data), the French dictionary will kick in when you look up a word while reading the book.
If that's not an issue for you, you can go ahead and delete the unwanted dictionaries. Or, you can play it safe, and simply create a collection for them. That way, they will still be available if needed, but they won't clutter the home screen.
Mike
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