It is expected that as a beginner, your to do your own research before questioning a standard practice, this is not to say that you asking questions is a wrong idea, but if you did a little search on DCA, you would understand the term is not only use for BTC, but with most investors using USD across their investment including stock and other trades, however if you decide to use your own local currency as the acronym for cost averaging, nobody is spoting you but don't forget that U.S dollars is the standard and most influential currency in the world at the moment.