These are the languages in which you can create Journeys and Tips on, as well as subsequent translations.
To simulate browser languages, you have a wider choice of languages. Check the article called "Simulating a Browser Language".
The selection of supported languages consists of selected languages according to ISO 639-1. In some special cases, the language can be determined also by the region (RFC 4646). The supported languages are listed alphabetically and written in the target language itself.
EN english US
EN english UK