When you use lang=xx bbcode then you finish it without parameter - only /lang.
If someone has set his default language then by default mod will language bbcode to whole his post and also to title. It is configurable - so user can disable it in editor if he wants to write some parts in different languages, and then he have to add bbcodes manually.
So right now users own their language bbcodes and don't have to set it - they just need to set default language in User CP