The TD;DR: I would classify it as a bug.
A bit of background:
The store for the product alerts always is determined as follows in
$store = $this->_website->getDefaultStore();
$storeId = $store->getId();
$templateId = Mage::getStoreConfig($templateConfigPath, $storeId);
'area' => 'frontend',
'store' => $storeId
The so it always uses the default store of the specified website.
The website in turn is set during the subscription. That is when a customer subscribes to notifications the current website ID is stored with the subscription record:
$model = Mage::getModel('productalert/price')
So even if the customer has registered in, lets say, the french store view, and has subscribed to the notifications in the french store view, if the default store for the website happens to be the english store view, that specifies which template will be used.
To fix the behavior, rewrite
Mage_ProductAlert_Model_Email::send() and use the customers store_id instead (for example).