CiviCRM 5.9.1 on Joomla 3.9.2 Problems with the language
Michael, Welcome to Stackexchange. The language will be available if you have downloaded and place l10n folder in civicrm. 

To configure CiviCRM to use an existing translation:


Download the translations distribution -
civicrm--l10n.tar.gz - from the CiviCRM Sourceforge
downloads site. This tarball contains all files for the latest
available translations.
Uncompress the tarball and you will see a directory called civicrm,
containing two subdirectories called l10n and sql. The l10n
directory contains a set of sub-directories named using their locale
codes. (Example: the Canadian French translation files will located
in /civicrm/l10n/fr_CA) 
Copy this l10n directory, together with all
    its subdirectories into your CiviCRM codebase root directory. (note
    if creating the l10n folder manually it should be ell-ten-n, not
    capital-I-ten-n) 


For Joomla this would be
   site_root/administrator/components/com_civicrm/civicrm. So if you
   wanted the en_GB language it would look like this

site_root/administrator/components/com_civicrm/civicrm/l10n/en_GB/


For Drupal this would be site_root/modules/civicrm. So if you wanted the en_GB language it would look like this site_root/modules/civicrm/l10n/en_GB/


Login to CiviCRM (as a user with "administer CiviCRM permissions").
Go to Administer » Localization » Languages, Currency, Location
You should now be able to see and select all the localization
options in the Default Language and drop-down menu. If you don't,
and instead can just see the default 'English (United States)'
option, check the preceding steps as the drop-down options are set
by the directories under your l10n directory. Also be sure to
confirm that the l10n directory is correctly named as l10n (that is:
el-ten-en and not eye-ten-en).


Cheers

Pradeep