I found out that there is one specific string in the core magento template base that somehow can't be translated.
./vendor/magento/module-sales/view/frontend/templates/order/items.phtml there is the heading "SKU":
<th class="col sku"><?php /* @escapeNotVerified */ echo __('SKU') ?></th>
If one tries to translate it with inline translations: there is no field to add customized text.
If one tries to translate it with i18n-CSVs: no change at all.
Everything else is translateable, but just not this single piece of string. Why is this and how could one translate this anyway? We are experiencing this Bug with Magento 2.1.11.