Try contacting Expath, founded by Americans, I think. They run workshops for new arrivals in Berlin. I did it some years ago and got a long list of useful hints. Maybe you can come to some arrangement with them before you get there.
I found a flat with www.immobilienscout24.de, and yes ads are usually in German, but many are in both German and English and maybe other languages or even English only if ad inserted by non-german speaker or someone who prefers an expat ( ie willing to pay a higher rent). Google translate is helpful and studying the ads with a map of the city allows you to develop familiarity with the parts of the city and a sense of what is available where at what price.
AirBnB could act as a temporary home while you look before you commit to a contract, or hostels if you do that sort of thing. Hostels vary. I recommend "The Cats Pajamas" near Hermannplatz but the area and the hostel have grown in popularity since I was in Berlin so availability may be uncertain.
Note that officially you should register when you arrive. This is not the same as registering with your own country's embassy if you want to do that. All inhabitants need to do this, not just foreigners. Germans do it when they move between cities. Foreigners are required to do it within 3 months of arrival, although I was told a short trip out the country resets the clock.
English is widely spoken in Berlin - too widely if you are trying to learn and practice German - but anything official will be undertaken in German. It is after all Germany.
Getting a flat with a formal contract can require that you have a German bank account; a bank account naturally requires that you have an officially registered address; and you need to have a flat before you can officially register its address. So it can be quite circular.
Be aware that many people sublet their flats or rooms in flats in Berlin. This is contrary to their rental contract but is quite common. You will not be able to register such an address and if you try there may be trouble if the landlord finds out. If you don't want to register and don't need a German bank account this won't be a problem though it may affect any insurance claim if something bad happens.