SQLSTATE[HY000] [2005] Unknown MySQL server host 'locahost' (1)

If you put “localhost”, that means that both MySQL and your Joomla site are on the same server. It is rarely the case.
In the database field, you must put the server where the Joomla MySQL database is located. You can find it in the configuration.php file in the Joomla root.

Hello,

This is what I find in the configuration.php file: var $host = ‘localhost’;

Is the WordPress site on the same server as the Joomla site?
It seems like it is not the case. In this case, in the host field, you must enter the name of the server where your Joomla database is located (the public name and not localhost).

This is what I get when I replace localhost with the name hostname of my server:

[fgj2wp] Couldn’t connect to the Joomla database. Please check your parameters. And be sure the WordPress server can access the Joomla database.
SQLSTATE[HY000] [2003] Can’t connect to MySQL server on ‘m15.siteground.biz’ (4)

If your WordPress host doesn’t have access to your Joomla MySQL database, the method is to export your Joomla database with PHPMyAdmin and import it on your WordPress database (with PHPMyAdmin too). After that, you can configure the plugin to point on the WordPress database (hostname and database fields).