Imagine a blog platform that's been carefully localised. Every button, menu and error message is in French. A French reader opens an article, and the article itself is in English.
Translating the interface doesn't translate the content. So far, the book has covered the first kind of text in an application, the text in your source code. This chapter covers the second kind, the content stored in your database. That includes blog posts, product descriptions, help articles, forum comments, and anything else someone wrote after the code shipped.
The two need very different solutions. Interface text belongs to you, changes when you deploy, and goes through your translators. Content might belong to your users, changes all the time, and might never be translated at all.