switching languages doesn't include switching currency because there are countries with the same language but different currencies. For example: English is spoken in the UK as well as in the USA. Same language, different currencies. But you can add to change Country and it automatically changes currency then.
Step 1: go to Settings and add the zone you want to sell under "shipping and delivery".
Step 2: Edit your theme "Header" and make sure you chose "Selection for Country/Region"