I finally found the solution.

I had to execute this sql request: DELETE FROM pg_proc WHERE proname='unaccent';
Because it seems like Odoo still detects the database as in unaccent mode.

Thanks Fred.

We use option unaccent = True in config file.

But also for each database you have to run this command

psql <database> -c "CREATE EXTENSION \"unaccent\"";

What I asked for was a way to avoid running this command for each created database. And it looks there is no way without editing odoo code.

I could be great if we could include this code in postgresql and get it running for each database creation.

Any ideas?

Start the server with the option --unaccent

/path/to/openerp-server.py --unaccent

Or add the option

unaccent = True

in the OpenERP configuration file... 

answer found here \https://www.odoo.com/fr_FR/forum/aide-1/question/how-to-search-without-accented-characters-1229