Define in which languages your content is available. Fields can be marked as localizable to manage them in multiple languages.
Some important facts:
A language can be marked as optional. When a field is marked as required you can keep the field empty for this language.
A language can have fallback languages. When you request ca content in a specific language and no value is available for a localizable field and the requested field we try the fallback languages in the defined order before we try the master language.