The device is not locked per se but you will only be able to download the content associated with whichever login (user) you have used to purchase the content. You can buy kindle books in English on both .de and .fr (perhaps not all titles but generally I have found it makes no difference-other than price) and have them delivered to your kindle device. If you have purchased books from both domains, then only one will show at a time. You can always log out and login with another user and download the content. Hope this helps and isn’t too confusing.