Enable the language switcher for the Farsi edition

Mirrors QuantEcon/lecture-python-programming#490, which adds the same
block to the English source. Lists all four editions with
current_language set to fa so this edition renders as active, and
injects the hreflang alternates for the set.

English stays first in the list because the theme uses languages[0] as
the hreflang x-default target.

Verified with a local build against the theme: the switcher lists all
four languages with fa active.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/_config.yml

@@ -32,6 +32,20 @@ sphinx:
     html_theme: quantecon_book_theme
     html_static_path: ['_static']
     html_theme_options:
+      languages:
+        - code: en
+          name: English
+          url: https://python-programming.quantecon.org
+        - code: fa
+          name: فارسی
+          url: https://quantecon.github.io/lecture-python-programming.fa
+        - code: fr
+          name: Français
+          url: https://quantecon.github.io/lecture-python-programming.fr
+        - code: zh-cn
+          name: 中文
+          url: https://quantecon.github.io/lecture-python-programming.zh-cn
+      current_language: fa
       authors:
         - name: Thomas J. Sargent
           url: http://www.tomsargent.com/