I recently released an app on Apples App Store and currently have it available in 2 languages, Swedish & English(U.S.). So my question is, what language will be presented to other English speaking countries since I picked English(U.S.)?. Do I have to add a description for all of them, for example, Australia & the U.K to avoid displaying the Swedish version of the app in the app store?
It depends on the default language you set for your app. If you picked English as a default language then all descriptions will be in English on every store.
And if you want to have a swedish description. Choose English as the default language and then add the swedish language in the country list in the top right corner (click the + button) and add the swedish description.