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/_locales/en/messages.json b/src/_locales/en/messages.json
index 433fd17..657b849 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -13,7 +13,7 @@
 	},
 	"contextmenu_title": {
 		"message": "Translate selection into...",
-		"description": "Title of the context menu that appears when a right click is done. Inside this parent menu there are the menus"
+		"description": "Title of the context menu that appears when a right click is done. Inside this parent item there are the target languages."
 	},
 	"contextmenu_title2": {
 		"message": "Translate selection into '$language$'",
@@ -27,35 +27,35 @@
 	},
 	"contextmenu_edit": {
 		"message": "Edit languages...",
-		"description": "Title of the option inside the 'Translate section into...' context menu."
+		"description": "Title of the item inside the 'Translate section into...' context menu which is used to go to the options page."
 	},
 	"options_welcome": {
 		"message": "Welcome!",
-		"description": "Title of the options page"
+		"description": "Title of the options page which welcomes users"
 	},
 	"options_introduction": {
 		"message": "Please, select the languages you want to show up in the \"translate\" menu.",
-		"description": "Introduction paragraph to the options pages"
+		"description": "Introduction paragraph in the options page preceding the language list."
 	},
 	"options_languageselectheader": {
 		"message": "Languages:",
-		"description": "Header of the language select option."
+		"description": "Header of the selected languages list."
 	},
 	"options_otheroptionsheader": {
 		"message": "Other options:",
-		"description": "Header of the options page."
+		"description": "Subheader in the options page before the options to choose where should the translation be shown.."
 	},
 	"options_tabsoption_1": {
 		"message": "Open Google Translate in a new tab for each translation.",
-		"description": "Option which defines how should the app behave in relation to tabs."
+		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in a new tab)."
 	},
 	"options_tabsoption_2": {
 		"message": "Open Google Translate in a unique tab and override the last translation instead of opening several tabs.",
-		"description": "Option which defines how should the app behave in relation to tabs."
+		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in the same tab used for the previous translation, or will open a new tab if there aren't any translations tabs open)."
 	},
 	"options_tabsoption_3": {
 		"message": "Open Google Translate in a panel (you need to activate <a href='chrome://flags/#enable-panels' id='panelsflag'>--enable-panels</a> flag). <sup style='color:red; font-weight: bold;'> EXPERIMENTAL</sup>",
-		"description": "Option which defines how should the app behave in relation to tabs."
+		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in a new panel)."
 	},
 	"options_savebutton": {
 		"message": "Save",
@@ -67,7 +67,7 @@
 	},
 	"options_addlanguage_addbutton": {
 		"message": "Add",
-		"description": "Add button in the footer of the languages list"
+		"description": "'Add' button in the footer of the languages list which is used to show a dialog with a list of potential languages to add. In the dialog, this message is also shown inside a button which is used to confirm the action."
 	},
 	"options_language_label": {
 		"message": "Language:",
@@ -75,11 +75,11 @@
 	},
 	"options_credits": {
 		"message": "Credits",
-		"description": "Title for the credits link and dialog"
+		"description": "Message for the link to the credits dialog, and also the title for the credits dialog, which shows a list of open source projects used inside the extension and a list of translators/contributors."
 	},
 	"options_credits_homepage": {
 		"message": "homepage",
-		"description": "Homepage link for an item in the credits. NOTE: put in in lowercase letters"
+		"description": "Text shown for links to go to each homepage of open source projects in the credits. NOTE: put in in lowercase letters"
 	},
 	"options_credits_by": {
 		"message": "by",
@@ -95,11 +95,11 @@
   },
 	"options_ok": {
 		"message": "OK",
-		"description": "OK button in the dialogs"
+		"description": "OK button in informative dialogs, which is used to close them (no action is done when pressing these buttons)"
 	},
 	"options_cancel": {
 		"message": "Cancel",
-		"description": "Cancel button in the dialogs"
+		"description": "Cancel button in the dialogs, to reject an action."
 	},
 	"notification_install_title": { "message": "Thanks for installing 'Translate Selected Text'" },
 	"notification_install_message": { "message": "Click this notification to set it up." },
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">