An easy way to start is by considering the same language countries that your product is originally sold in and website is written in. So if your product sells well in North America, what are the other English speaking countries you target? Start with some top performing Facebook ads, duplicate those, target them to that country and go. See what happens.
When you are working with a same language market, the best solution is to use a currency converter on your site. When you are a Shopify Plus member part of your monthly cost is cloning up to 10 stores. Creating a new store would be beneficial when you are then moving into different languages. Using a Java Script to translate a site can get messy.