Translation after submitting form on block (and error/nfo/success messages)

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Hi,

I've a block that will show the correct language translations on first load. The block contains a form.
When I post the block form, it will handle it in action_save action, but it is not in the correct language anymore! What do I have to do to get this in the correct language?

Also, how do I display error messages from the block? This didn't work:

public function action_save($bID) {
    // Set message.
    $this->set('message', t('This is a message'));
     // Show view?
    $this->view();
}


Update:
First load will use the correct (site)locale: nl_NL. After submitting, I see that the site locale is en_US... Something isn't working correctly...

SnefIT