I have an android app on the Play Store with en-US as the default language, and a few other languages, such as es-ES. But Play Store have 3 es languages (South America, Spain and US). If I only have es-ES translation, an user using es language from South America will see my es-ES or he will see the main language (en-US)?
Since API Level 24, Android will automatically try to find better fallbacks if the user's specific locale is not supported. So if you are a Spanish speaker from Colombia, Android will try to find Latin American Spanish language settings (es-419). If that is not found, it will the look for other Spanish locales and will eventually come to es-ES if that is the only one available.
You can read more here.