In our case we tried to set 'en' as default language but it did not work so far. In the global.ini.php we found the following lines

; if you want all your users to use Piwik in only one language, disable the LanguagesManager
; plugin, and set this default_language (users won't see the language drop down)
default_language = en

Also with LanguagesManager disabled (removed from global.ini.php), Piwik always switches Language and displays the language drop down in the user settings.

Can anybody reproduce this?