If you want your editors to be able to write content in a particular language, the language must be added and activated for use. This is done under Manage Website Languages.
When you open Manage Website Languages, all the languages added to the website are shown in a long list and you can immediately see which languages are available and whether they are active. Change the settings for a language by clicking the language’s name. You can adjust the order of the various languages using the arrows to the left of the language.