What is the difference between "Language: Not specified" & "Language: Not applicable"? What is the difference between Language: Not specified and Language: Not applicable in Drupal 8 In the context of multilinguality ?

There was no such distinction in Drupal 7 (and 6, IIRC) and it seems to me a bit redundant in comparison to the Drupal 7 method of Language: Natural vs Language: name. I also didn't find any documentation about this in a quick Google search about the difference and thought it would be nice if we would have an explanation here.
Not applicable means that we know that the very concept of Human languages doesn't apply to this textual piece of content. A relevant example for such content would be Binary code.

For more see http://www.w3.org/International/questions/qa-no-language#nonlinguistic.

Not specified means we don't know if the text might or might not include human-language content.

For more see http://www.w3.org/International/questions/qa-no-language#undetermined.

(Both "see more" links are taken from the code documentation in \Drupal\Core\Language\LanguageInterface which explains those constants quite well).



In general, both options makes no difference to how content is displayed, but they let you treat differently to content on your site if you'd wanted to (in views, custom code, ...)