I've changed how phpBB displays thousands by adding the number_format() to the totals. However, there are some which needed a bit of playing with before it would work.
For the forum stats at the bottom of the index page:
Before
'TOTAL_POSTS' => sprintf($user->lang[$l_total_post_s], $total_posts),
'TOTAL_TOPICS' => sprintf($user->lang[$l_total_topic_s], $total_topics),
'TOTAL_USERS' => sprintf($user->lang[$l_total_user_s], $total_users),
After:
'TOTAL_POSTS' => sprintf(str_replace('%d', '%s', $user->lang[$l_total_post_s]), number_format($total_posts)),
'TOTAL_TOPICS' => sprintf(str_replace('%d', '%s', $user->lang[$l_total_topic_s]), number_format($total_topics)),
'TOTAL_USERS' => sprintf(str_replace('%d', '%s', $user->lang[$l_total_user_s]), number_format($total_users)),
