Product names are randomly translated in Magento widget

Question

I've implemented a custom product slider in my Magento store. Sometimes the product titles in the slider are in English instead of Dutch on the Dutch storeview. When I empty the cache everything works normal (titles back to Dutch) for about 1-2 hours and then after period of time the product titles go back to English.

This is the code that is used for the product collection in the sliders:

protected function _getProductCollection()
{
    $productCollection = null;

    $category = $this->getCategory();
    if ($this->getCategory() != null && $category->getId()) {
        $productCollection = $category->getProductCollection()
            ->addStoreFilter()
            ->setPageSize(11)
            ->setCurPage(1);

        Mage::getSingleton('catalog/product_visibility')
            ->addVisibleInCatalogFilterToCollection($productCollection);

        $this->_addProductAttributesAndPrices($productCollection);
    }

    return $productCollection;
}

Sidenote: We've recently implemented a hreflang module in our store. After this implementation the problems seem to have started.