I realize that this thread is a bit old, but I’d like to toss in some additional information.
Richard Webb, the author of 80/20 Japanese recommends redefining how the particle は is expressed in English. は is the topic marker, but in English we don’t often refer to topics directly, usually only referring to the subject. For a sentence like 「これは何ですか?」, translating は as ‘is’ to get the sentence “what is this?” is acceptable since it does get the meaning across properly, but it does oversimplify what is actually going on. After all, in the Japanese sentence, ‘this’ is marked as the topic, whereas in the English translation it is actually a subject, and this causes problems in some other translations. Richard suggests instead defining は as something along the lines of “When talking about…”, which, while more cumbersome and unnatural in English, more accurately follows the Japanese grammar. With this in mind your example sentence, 「それはどれですか?」, now translates to “When talking about that/those, which?”, which is much closer to the true meaning of the original Japanese sentence.
If you’ve got 80/20 Japanese yourself, the section on how は best translated into English is on page 302 if you’d like more detail and examples.