In [dotorg11868] I've updated the bbPress plugin directory to store the plugin slugs in usermeta in addition to bbPress topic_ids.
get_user_metadata( $user_id, 'plugin_favorites', true ) will return an array of plugin slugs which have been marked as favorited in the bbPress directory by the user.
A data migration hasn't occurred, but will happen if you favorite/unfavorite a plugin via bbPress. We'll need to run a migration before this can be marked fixed.
Previously the plugin favorites were storing bbPress topic id's in plugin_favorite, GlotPress would benefit from the new slug storage. The Theme Directory also stores favorites in theme_favorites as theme slugs.