Change the title of spoiler button by state

The title will give the precise representation of the current state. It
would be helpful for blind people.

app/javascript/mastodon/locales/ko.json

@@ -61,7 +61,8 @@
   "compose_form.publish": "툿",
   "compose_form.publish_loud": "{publish}!",
   "compose_form.sensitive": "이 미디어를 민감한 미디어로 취급",
-  "compose_form.spoiler": "텍스트 숨기기",
+  "compose_form.spoiler.marked": "Text is hidden behind warning",
+  "compose_form.spoiler.unmarked": "Text is not hidden",
   "compose_form.spoiler_placeholder": "경고",
   "confirmation_modal.cancel": "취소",
   "confirmations.block.confirm": "차단",