What is the difference between "Language: Not specified" & "Language: Not applicable"?
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, ...)