When installing a language from the administrator environment ie. Dutch, following message appears.

Error Unable to detect manifest file.

When trying the direct download on http://joomlacode.org/gf/project/jtranslation3_x/frs/?action=FrsReleaseBrowse&frs_package_id=6406

after some while it shows

Unhandled PropelException [wrapped: Could not connect [User Info: host=xxx.xxx.xxx.xxx dbname='xxxxxx' user='xxxxxx' password='*********'] [User Info: Array]]
#0 /opt/gforge.git/lib/core/om/BaseLanguagePeer.php(203): Propel::getConnection('gf5')
#1 /opt/gforge.git/lib/core/om/BaseLanguagePeer.php(183): BaseLanguagePeer::doSelectRS(Object(Criteria), NULL)
#2 /opt/gforge.git/lib/core/om/BaseLanguagePeer.php(165): BaseLanguagePeer::doSelect(Object(Criteria), NULL)
#3 /opt/gforge.git/www/gf.php(113): BaseLanguagePeer::doSelectOne(Object(Criteria))
#4 {main}

I replaced the IP and such for xxxx, but they do show.

I am on 3.9.22
PHP 7.3.24
Apache 2.4.45
MySQL (mariadb) 10.1.47

I am not the only one: https://forum.joomla.org/viewtopic.php?f=711&t=982856