explicit is better than implicit

apps/amo/urlresolvers.py

@@ -210,11 +210,9 @@ def url_fix(s, charset='utf-8'):
 
 def lang_from_accept_header(header):
     # Map all our lang codes and any prefixes to the locale code.
-    langs = [(k.lower(), v) for k, v in
-             sorted(settings.LANGUAGE_URL_MAP.items())]
-    # Start with prefixes so any real matches override them.
-    lang_url_map = dict((k.split('-')[0], v) for k, v in langs)
-    lang_url_map.update(langs)
+    lang_url_map = settings.SHORTER_LANGUAGES.copy()
+    lang_url_map.update((k.lower(), v) for k, v in
+                        settings.LANGUAGE_URL_MAP.items())
 
     # If we have a lang or a prefix of the lang, return the locale code.
     for lang, _ in parse_accept_lang_header(header.lower()):

lib/settings_base.py

@@ -122,6 +122,11 @@ AMO_LANGUAGES = (
     'zh-CN', 'zh-TW',
 )
 
+# Explicit conversion of a shorter language code into a more specific one.
+SHORTER_LANGUAGES = {
+    'en': 'en-US', 'ga': 'ga-IE', 'pt': 'pt-PT', 'sv': 'sv-SE', 'zh': 'zh-CN'
+}
+
 # Not shown on the site, but .po files exist and these are available on the
 # L10n dashboard.  Generally languages start here and move into AMO_LANGUAGES.
 HIDDEN_LANGUAGES = ('cy', 'sr', 'sr-Latn', 'tr')