[ticket/12222] Replace hardoded comma with translatable separator

Replace hardoded comma with translatable separator
in forumlist_body.html.

PHPBB3-12222
Merge branch 'ticket/12222' of github.com:rxu/phpbb3 into ticket/rxu/12222

# By rxu
# Via rxu
* 'ticket/12222' of github.com:rxu/phpbb3:
  [ticket/12222] Replace hardoded comma with translatable separator
Merge branch 'ticket/rxu/12222' into develop

* ticket/rxu/12222:
  [ticket/12222] Replace hardoded comma with translatable separator