I18n - Default locale is overridden when English ('en') is added

Really strange problem; I have internationalization enabled and have added 3 languages: Dutch, French and English. When I go to Realm Settings > Theme and set a default locale, that locale is ignored and the language is based on the browser’s language. When I remove ‘en’ (English) from my locales, so that I only have ‘nl’ (Dutch) and ‘fr’ (French), the default locale is not ignored and is correctly set when I go to the login-page. Has someone had this before? Why is this happening?