The word ersatz has been used in English since the late 19th century http://www.etymonline.com/index.php?term=ersatz. Naturally, the pronunciation differs from the High German pronunciation (voiced and unvoiced pronunciations of the 's' are recorded, and the 'r' is pronounced only in rhotic dialects), and the meaning may also be different.
It is a rarer word compared with the synonyms "replacement" or "substitute", it also carries a connotation of "artificiality" or "of poor quality", that its synonyms don't have. Wiktionary has list of quotations https://en.wiktionary.org/wiki/ersatz. There are some particular examples for which ersatz is the normal choice of word: ersatz coffee for roast chicory is one example.