SPIP offers the possibility, as presented in the configuration page table, of selecting the URL naming convention that you would like to employ (You must eliminate —comment our or delete — the $type_urls variable in your mes_options.php file, in order to make that configuration table be visible.).
In order for your URL naming convention to be taken into consideration, it is normally common for most installations that you use an htaccess file:
Start with the htaccess.txt file supplied with SPIP as a base example, and which works with most ISP hosting services for the types of URL naming offered.
To activate it, you must copy it back into the root directory as .htaccess. It is highly recommended to open it before copying it to check certain elements of the configuration ( warning: any modification of the .htaccess file can lead to serious impacts on the operation and performance of your web site.)
You should then test the validity of these new URL addresses by calling the "View online" page link for an article, an author, a news article, a section, etc.
You can find more information about "how to use personalised URLs" in this article of the documentation.