Bug reproduction steps: login → change your language to Deutsch → open the site in incognito/in some other browser then your current browser/ in your smartphone, with browser set to English, and login again you will see the language set to Deutsch in your profile but the interface will be in English.
This is unintended behaviour. When users select a language which differs from their browser language we should respect that and always update the interface to display in the language selected in their preferences.