If you follow a link with a different interface language set, it will override the one set in your profile.
E.g following this link will set it to Korean:
It will also add languages:
Following this link will 1, set your interface language to German and 2, add Portuguese to your list of active languages.
Removing languages is also tricky. If you follow this link, see that Portuguese is added, then try to remove it in the settings without changing back to one of the languages you are actually learning, it will re-add it since the language code is still in the URL when you navigate away. E.g:
So you need to switch language lessons in order to remove it.
It is easy to change the interface language back regardless of the language because the settings page is really small. But it gets really tiring if you follow form posts and discussions, and constantly have to manipulate URLs and clean up added languages.
Yes, that’s what happens at the moment. That’s not really a bug, but if you open an URL with a language code, it will activate that study language. I don’t think anything will change there in the near future, but there’s definitely room for improvement and we’ll see what we can do in future updates.