After installing Joomla WITHOUT sample data, you first have to create Content Languages for each language.
Once these are defined, they will be available all over Joomla User Interface.
This can be done even if the site language pack is not yet installed.
You will just have to make sure that the desired site pack is installed and enabled before launching the site.
==>Go to "Language Manager" ->"Content" tab after login in the back-end.
Enter parameters for each content language.
The URL Language Code, the Language Tag and the Image Prefix shall be unique.
The Language Tag is the language pack one (fr-FR, en-GB, de-DE, etc.).
The Title is used in the "langswitcher" module (1.6.1 and 1.6.2). The Title Native in later versions.