docs: add documentation about internationalization from a user perspective

While there is already documentation about creating and updating
translations (developer perspective), there was nothing for the user/admin
of Kallithea.
Since the handling in TurboGears2 is different, this is a good time to add
it.

docs/setup.rst

@@ -67,6 +67,21 b' You are now ready to use Kallithea. To r'
   repositories.
 
 
+Internationalization (i18n support)
+-----------------------------------
+The Kallithea web interface is automatically displayed in the user's preferred
+language, as indicated by the browser. Thus, different users may see the
+application in different languages. If the requested language is not available
+(because the translation file for that language does not yet exist or is
+incomplete), the language specified in setting ``i18n.lang`` in the Kallithea
+configuration file is used as fallback. If no fallback language is explicitly
+specified, English is used.
+
+If you want to disable automatic language detection and instead configure a
+fixed language regardless of user preference, set ``i18n.enabled = false`` and
+set ``i18n.lang`` to the desired language (or leave empty for English).
+
+
 Using Kallithea with SSH
 ------------------------