Fix several bugs

- Fix: when installing the extension, the default language was added
  even if it didn't exist in isoLangs.
- Also, the default language now considers 'zh-CN' and 'zh-TW' as
  possible candidates.
- Fix: minor CSS issue in the options page.
- Unknown languages are deleted (or replaced with a known replacement)
  when updating the extension.
- Other minor changes.

Change-Id: Id6fc8233255cef2fc67185e1114ca86fdc56bc5a
diff --git a/src/css/options.css b/src/css/options.css
index 9aa7b75..7190881 100644
--- a/src/css/options.css
+++ b/src/css/options.css
@@ -22,7 +22,7 @@
 
 #languages_container {
   width: 300px;
-  height: 365px;
+  height: 250px;
   border: 1px solid #ccc;
   background-color: #E3F2FD;
   overflow: auto;
@@ -185,7 +185,7 @@
   font-weight: bold;
 }
 
-dialog .action_buttons {
+dialog#credits_dialog .action_buttons {
   border-top: 1px solid #ccc;
   padding: 1em;
   text-align: right;