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->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.