Clarify i18n strings descriptions

This change clarifies i18n string descriptions and changes one string of
the "Add language" dialog in the options page ("OK" button --> "Add").

Change-Id: I4085085ddaa19ab9b6c8a526c1001b7c3ccb2672
diff --git a/src/options.html b/src/options.html
index ade625c..ef2efc5 100644
--- a/src/options.html
+++ b/src/options.html
@@ -42,7 +42,7 @@
     </div>
     <div class="action_buttons">
       <button id="languages_add_cancel" data-i18n="cancel"></button>
-      <button id="languages_add_ok" data-i18n="ok"></button>
+      <button id="languages_add_ok" data-i18n="addlanguage_addbutton"></button>
     </div>
   </dialog>
   <dialog id="credits_dialog">