I don’t know how to change language?

You need a .MO file for the language you desire and you need to reference that language in your wp-config.php file.

So the french language as a file called wp-includes/languages/fr_FR.mo and has this in wp-config.php
define ('WPLANG', 'fr_FR');

Resource:
WordPress_in_Your_Language
Translating_WordPress

Even if radiox didn′t say a word if MichaelH′s comment helped, it helped me!

THANKS MichaelH!