Language detection settings (admin/config/regional/language/configure) determine which language UI is presented to a visitor. For example, if you set path prefixes as the first detection method, then example.com/fr will automatically take any visitor to the French UI.
Now, what happens then when someone visits example.com ? There is no path prefix here. If there is another detection method such as Determine the language from the browser's language settings, and the visitor's browser is Dutch, then in your case, this visitor will go to example.com/nl. When all detection fail, the visitor will go to the default site language. And in your case, that will be English.
If you don't need English for visitors, then perhaps you should set the default language to French or Dutch. WARNING - it is not recommended to change the default language after you have created content because it will mess up translations.