When you sell a novel to a publisher, you sell them specific right to publish in certain languages and certain countries. Whatever rights you don't sell, you retain and can sell to someone else. Usually publishers want to buy all rights, and usually writers want to sell only limited rights, since they can make more money by selling the rights individually in different countries.
Unless the writer is very famous or has a very good agent, they probably lose this argument with the publisher and sell all rights. This means they get less money for what are called "secondary" rights -- translations and sales in other countries. So it matters where you sell the primary rights. A lot of Canadian authors, for instance, end up earning less money on their Canadian sales because their American publisher considers Canadian right to be secondary and takes their cut of the profits. So the calculation becomes, do I sell primary rights in Canada, where, as a Canadian I may sell more books, or do I sell primary rights in the US which is a market 10 times the size? You might have the same dilemma between Greek and English markets as primary and secondary. But you shouldn't take the above as gospel. It is based on hearsay and could be out of date. You should consult a good agent.
My guess is that the fact that you wrote half in English and half in Greek might be a point that the publicity department might want to use for promotional purposes, but otherwise isn't going to make any difference. The only exception might be that if the book becomes very famous, or becomes of academic interest, then at some point in the future the publisher might think about doing a dual language edition. Maybe a small Greek literary publisher might be interested in doing a dual language edition right off the bat, but that would appeal to a very small market. Then again, you can never quite tell when a novelty item, like a novel written in two languages, will appeal to the public's imagination, so who knows.
