Useful Information
Language
Compared to other areas of Poland, English is pretty widely-spoken in Warsaw. Any establishment that ever has any tourists will have somebody on the staff who can talk something that sounds like English. Outside of the tourist centre, and in dealing with officials such as the police, you are likely to find more trouble if you don't speak Polish. You are never far from an English speaker however, and most are more than willing to help translate. Bear in mind that Polish is a phonetically-spelled lanuage with somewhat different sounds to those normally found in English, so Polish English speakers often pronounce words strangely, and may have a little trouble understanding suprisingly common words when you speak. Just try a different word or pronunciation instead.
German and Russian are less widely spoken in Warsaw than in other parts of Poland, though they may be worth a try as a last resort.
Shop opening
Shops in Warsaw are open broadly western hours, from about 9 or 10am to between 6 and 8 at night. A wide range of shops is open at the weekend, although they tend to close a couple of hours earlier on Sundays.
Most of the large city centre shops are located between Nowy Swiat and the Palace of Culture. There are a range of bookstores, supermarkets and boutiques in this area. In particular Traffic, located in the dead centre of this area, has several floors of books, with international and Polish DVDs and music in the basement. The top floor of Traffic is a stylish club with live music most nights of the week.
There are a number of out-of-town shopping centres. In particular Arkadia is a favourite amongst the ex-pat community. This has the usual range of shops you would expect at any out-of-town mall.