How the primary language works as the fallback
Apple's own example: English is primary and you add French. The French listing shows to people whose device language is French, and in storefronts that support French but not English. If nothing matches someone's language setting, Apple uses the next most relevant localization, and everywhere else the primary language.
English (U.S.) is listed in two storefronts, the US and Japan, while English (U.K.) appears in 172 of 175. A US-first app can add English (U.K.) as a second English listing, with British spelling and its own keywords for the UK, India and the other storefronts that list it.
To change the primary language, pick another one in the Primary Language menu under App Information and click Save. Apple only offers a language you've already shipped on a previous version, approved by App Review, with approved screenshots in your current primary language's sizes on every platform (and on each custom product page).