The default language is the language that is used as a fallback when a text is not available in the current language. For example, if a user has Dutch as their language and the application has only partly been translated from English to Dutch, you will see English texts in places where Dutch texts are not available.
Setting the language of MxAdmin can be done by creating a data grid and data view on User (not Account) and editing the language of the MxAdmin user from there.