Drupal 8 blocks content is not being translated I've searched high and low but can't seem to find any question that has the same problem. We're experiencing blocks that have content that is not being translated. For example, a custom block that has a simple translatable string and is being translated into a different language, it will simply always show the source language. What's up with that? 

$build['test_content']['#markup'] = $this-&gt;t('Test NL'); 


Edit: For the record; we have translated this string into the target language but still doesn't show up on the corresponding language.
You need to navigate to admin/config/regional/translate. Try to search for "Test NL" in the input field. It will give you the string on the left side along with a text-field to the right. Enter the translation for the "Test NL" in the text field. and click save translations. 

You are done. Clear the cache and check the interface.