For multilingual installations you also have to install the second languages from the scratch again. Proceed as follows:
If you want to use the second language as usual without own configuration, simply delete the config.php from the directory of the second language.
If want to have a configurable second language, leave the config.php in the directory of the second language, login and configure the second language to your needs.
Under "language default", the language of the main installation has to be activated. Their own language, the second language recognizes itself.
And don't forget:
!!! CHANGE YOUR PASSWORD !!!