[fix] get correct locale with country from browser

Some of our interface locales include uppercase country codes,
which are separated by `_` instead of the more common `-`.
Also, a browser's `Accept-Language` header could be in lowercase.

This commit attempts to normalize those cases so a browser's
language+country codes can better match with our locales.

This solution assumes that our UI locales have nothing more than
language and optionally country. If we ever add a script specific
locale like `zh-Hant-TW` this would have to change to accomodate
that, but the idea would be pretty much the same as this fix.

tests/unit/test_webapp.py

@@ -198,6 +198,20 @@ class ViewsTestCase(SearxTestCase):
             result.data
         )
 
+    def test_browser_locale(self):
+        result = self.app.get('/preferences', headers={'Accept-Language': 'zh-tw;q=0.8'})
+        self.assertEqual(result.status_code, 200)
+        self.assertIn(
+            b'<option value="zh_TW" selected="selected">',
+            result.data,
+            'Interface locale ignored browser preference.'
+        )
+        self.assertIn(
+            b'<option value="zh-TW" selected="selected">',
+            result.data,
+            'Search language ignored browser preference.'
+        )
+
     def test_stats(self):
         result = self.app.get('/stats')
         self.assertEqual(result.status_code, 200)