Magento 2 – How to Translate Order Email

Found out the issue. I should not have been using the code sample from app/design/frontend/base/default/template/email/order/invoice/items.phtml but should have used this snippet of code

<tfoot>
    <?php echo $this->getChildHtml('order_totals')?>
</tfoot>

from here app/design/frontend/base/default/template/email/order/items.phtml

In addition to that, my sales.xml file had to be modified, specifically the <sales_email_order_shipment_items> block to mimic what is found in <sales_email_order_items>:

<sales_email_order_shipment_items>
    <block type="sales/order_email_items" name="items" template="email/order/items.phtml">
        <action method="addItemRender"><type>default</type><block>sales/order_email_items_order_default</block><template>email/order/items/order/default.phtml</template></action>
        <action method="addItemRender"><type>grouped</type><block>sales/order_email_items_order_grouped</block><template>email/order/items/order/default.phtml</template></action>
        <block type="sales/order_totals" name="order_totals" template="sales/order/totals.phtml">
            <action method="setLabelProperties"><value>colspan="5" align="right" style="padding:3px 9px"</value></action>
            <action method="setValueProperties"><value>align="right" style="padding:3px 9px"</value></action>
            <block type="tax/sales_order_tax" name="tax" template="tax/order/tax.phtml">
                <action method="setIsPlaneMode"><value>1</value></action>
            </block>
        </block>
    </block>
    <block type="core/text_list" name="additional.product.info" />
</sales_email_order_shipment_items>

Ok, I just solved it thanks to @nobodypb https://github.com/magento/magento2/issues/3805#issuecomment-225160290

Just add the \Magento\Framework\App\AreaList dependency. And add this:

$areaObject = $this->_areaList->getArea(\Magento\Framework\App\Area::AREA_FRONTEND);
$areaObject->load(\Magento\Framework\App\Area::PART_TRANSLATE);

It works fine with my module! :D