Ariane,
what you have to do, depends on the access, your host granted to you.
If you are allowed to login as host, do so and go to host menu, item languages, if not, go to admin menu, item languages.
choose install language pack, select the zip file you have downloaded previously and add it to the upload list. choose upload, and the complete package gets installed.
Afterwards, if desired goto portal settings and set default language.
if desired, go to your profile and set your personal default language.
Sebastian