The language dropdown in the footer and navbar changes the interface language, and so does the Language select in Settings → Profile. Your country is set on the choose-region page. Currency isn't a separate choice: it follows whichever country you pick.
Changing the language
Open the language dropdown, or go to Settings → Profile.
Pick a language. The page reloads in it, and the choice sticks for next time.
Your country stays where it was.
Changing your country and currency
Go to the choose-region page. It's linked from the footer, from Settings → Profile, and from the banner that suggests a region.
Pick a country. They're grouped by region, and each one shows its currency code.
Wishpicks switches to that country, and the currency comes with it.
Each country maps to exactly one currency (US to USD, Germany to EUR, the UK to GBP), which is why there's no currency picker of its own. Your language stays where it was.
Moving into certain countries can also switch your Product news emails off until you opt in.
Prices aren't converted
Wishpicks doesn't recalculate an item's price into your currency. Every wish keeps the currency it was added in, so one list can show £, €, and ₴ side by side. That's expected, not a bug. Your country currency sets the formatting and fills in for items that arrived without a currency of their own.
Good to know
Wishpicks comes in English, Ukrainian, Polish, Russian, German, Spanish, French, Italian, and Portuguese (both European and Brazilian).
Related
"Why do prices on my wishlist show in different currencies?"
"How do I change my notification and email preferences (and turn off emails)?"