Yes, you do need to restart the spell checker when you switch 
dictionaries, but it's not too inconvenient in practice.

As you know, I've ran into issues with hunspell, which I described in 
this thread 
<http://lists.gnu.org/archive/html/help-gnu-emacs/2016-03/msg00107.html>.

-- 
Best Regards,
Nikolay Kudryavtsev