After you installed your preferred language you have to create a entry for this language in the Language Manager in the Content Section. Go to the Extensions->Language Manager->Content and click on "New".

Complete the fields shown in the image. The "Title" maybe the English version of the language, the "Title Native" is the translated version of the language. "URL Language Code" and the "Image Prefix" are the first 2 letters of the ISOCODE for the language. "Language Tag" is the complete ISOCODE of the language. If you don't like the letters you could take any string you like.

IMPORTANT: The Language Tag must match the installed language package, so if you install the jgerman translation you have to use "de-DE" as translation Tag and not "de-AT" or "de-CH".