By default, the currency displayed on your project is determined by its country of origin. That means if your banking and identity information is based in, say, Canada, your project goal and reward costs will automatically be displayed in CAD. If you’re in the UK they’ll be in GBP, and so on.
Kickstarter offers the option to set a preferred currency to be displayed when browsing projects. However, there is no way to choose a preferred display currency for your projects as this will depend on each person’s settings. Furthermore, backers’ pledges will be collected in the project’s native currency, not their preferred display currency.