MYSQL: Display NULL value when foreing key is NULL
I suggest you use the ANSI join that was introduced in SQL92. If you use a LEFT JOIN I think you achieve what you want: 

SELECT xs_bugtracking.id, product
     , xs_bugtracking.title AS title
     , status
     , xs_bugtracking.descr AS descr
     , assignee_tbl.user_nicename AS assignee
     , reported_tbl.user_nicename AS reported_by
     , importance
     , FROM_UNIXTIME(create_date) AS 'create_date'
     , FROM_UNIXTIME(create_date) AS 'modify_date' 
FROM xs_bugtracking
JOIN xs_products
    ON xs_bugtracking.product = xs_products.name 
LEFT JOIN wp_users AS assignee_tbl
    ON xs_bugtracking.assignee = assignee_tbl.ID
JOIN wp_users AS reported_tbl 
    ON xs_bugtracking.reported_by=reported_tbl.ID 
WHERE xs_products.lang='en'