Create a Joomla menu item to Kunena with language set to all. The URLs are hardcoded into the auto action at this time as CB Forums currently does not process its own activity (it will in a future release after CB Activity 4.0.0 API rewrite). You can modify the URL that gets stored by editing the "Kunena - Create" system action and saving a copy with Links > URL adjuted to whatever you like. Specifically to what what you're wanting you'd change it as follows.
FROM:
Code:
[cbpkb:ignore][cb:config param="live_site" /][/cbpkb:ignore]/index.php?option=com_kunena&view=topic&catid=[var3_message_catid]&id=[var3_message_thread]&mesid=[var3_message_id]
TO:
Code:
[cbpkb:ignore][cb:config param="live_site" /][/cbpkb:ignore]/index.php?option=com_kunena&view=topic&catid=[var3_message_catid]&id=[var3_message_thread]&Itemid=342&lang=en
This all works perfectly fine in my local tests and demo site.