[@netweb Keymaster 13 years, 9 months ago]

You can also use https://codex.bbpress.org/bbp_show_lead_topic/ as used here on bbpress.org to use a different template for the ‘lead topic’.

An alternative to your str_replace method is to make your own language translation. https://codex.buddypress.org/developer/extending-buddypress/customizing-labels-messages-and-urls/