diff --git a/src/internationalization/PhutilLocale.php b/src/internationalization/PhutilLocale.php
--- a/src/internationalization/PhutilLocale.php
+++ b/src/internationalization/PhutilLocale.php
@@ -30,11 +30,14 @@
    * For locales like "English (Great Britain)", missing translations can be
    * sourced from "English (US)".
    *
+   * Languages with no other fallback use en_US because that's better
+   * than proto-English for untranslated strings.
+   *
    * @return string|null Locale code of fallback locale, or null if there is
    *                     no fallback locale.
    */
   public function getFallbackLocaleCode() {
-    return null;
+    return 'en_US';
   }
 
 
diff --git a/src/internationalization/locales/PhutilCzechLocale.php b/src/internationalization/locales/PhutilCzechLocale.php
--- a/src/internationalization/locales/PhutilCzechLocale.php
+++ b/src/internationalization/locales/PhutilCzechLocale.php
@@ -26,5 +26,4 @@
 
     return $plural;
   }
-
 }
diff --git a/src/internationalization/locales/PhutilFrenchLocale.php b/src/internationalization/locales/PhutilFrenchLocale.php
--- a/src/internationalization/locales/PhutilFrenchLocale.php
+++ b/src/internationalization/locales/PhutilFrenchLocale.php
@@ -12,5 +12,4 @@
   public function getLocaleName() {
     return pht('French (France)');
   }
-
 }
diff --git a/src/internationalization/locales/PhutilGermanLocale.php b/src/internationalization/locales/PhutilGermanLocale.php
--- a/src/internationalization/locales/PhutilGermanLocale.php
+++ b/src/internationalization/locales/PhutilGermanLocale.php
@@ -12,5 +12,4 @@
   public function getLocaleName() {
     return pht('German (Germany)');
   }
-
 }
diff --git a/src/internationalization/locales/PhutilKoreanLocale.php b/src/internationalization/locales/PhutilKoreanLocale.php
--- a/src/internationalization/locales/PhutilKoreanLocale.php
+++ b/src/internationalization/locales/PhutilKoreanLocale.php
@@ -12,5 +12,4 @@
   public function getLocaleName() {
     return pht('Korean (Republic of Korea)');
   }
-
 }
diff --git a/src/internationalization/locales/PhutilPortugueseBrazilLocale.php b/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
--- a/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
+++ b/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
@@ -13,4 +13,13 @@
     return pht('Portuguese (Brazil)');
   }
 
+  public function getFallbackLocaleCode() {
+    // Phabricator does not support bidirectional
+    // fallbacks (pt_BR -> pt and pt -> pt_BR simultaneously)
+    // since Translatewiki pt_PT "Portugese" without a country
+    // it makes slightly more sense to fall back in this direction
+    // than the other one
+    return 'pt_PT';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilPortuguesePortugalLocale.php b/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
--- a/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
+++ b/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
@@ -13,4 +13,14 @@
     return pht('Portuguese (Portugal)');
   }
 
+  public function getFallbackLocaleCode() {
+    // Ideally this would be pt_BR but Phabricator does not support
+    // bidirectional fallbacks (pt_BR -> pt and pt -> pt_BR simultaneously)
+    // since Translatewiki calls pt_PT "Portugese" without a country
+    // it makes slightly more sense to fall back in the other direction
+    // In the mean time return `en_US` so users don't see Proto-English
+    // unncecessarily
+    return 'en_US';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilSimplifiedChineseLocale.php b/src/internationalization/locales/PhutilSimplifiedChineseLocale.php
--- a/src/internationalization/locales/PhutilSimplifiedChineseLocale.php
+++ b/src/internationalization/locales/PhutilSimplifiedChineseLocale.php
@@ -13,4 +13,13 @@
     return pht('Chinese (Simplified)');
   }
 
+  public function getFallbackLocaleCode() {
+    // Ideally this would be zh_Hant but Phabricator does not support
+    //  bidirectional fallbacks
+    // (zh_Hant -> zh_Hans and zh_Hans -> zh_Hant simultaneously)
+    // arbitrarily choose to fall back in the other direction instead
+    // In the mean time return `en_US` so users don't see Proto-English
+    return 'en_US';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilSpanishSpainLocale.php b/src/internationalization/locales/PhutilSpanishSpainLocale.php
--- a/src/internationalization/locales/PhutilSpanishSpainLocale.php
+++ b/src/internationalization/locales/PhutilSpanishSpainLocale.php
@@ -12,5 +12,4 @@
   public function getLocaleName() {
     return pht('Spanish (Spain)');
   }
-
 }
diff --git a/src/internationalization/locales/PhutilTraditionalChineseLocale.php b/src/internationalization/locales/PhutilTraditionalChineseLocale.php
--- a/src/internationalization/locales/PhutilTraditionalChineseLocale.php
+++ b/src/internationalization/locales/PhutilTraditionalChineseLocale.php
@@ -13,4 +13,8 @@
     return pht('Chinese (Traditional)');
   }
 
+  public function getFallbackLocaleCode() {
+    return 'zh_Hans';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilUSEnglishLocale.php b/src/internationalization/locales/PhutilUSEnglishLocale.php
--- a/src/internationalization/locales/PhutilUSEnglishLocale.php
+++ b/src/internationalization/locales/PhutilUSEnglishLocale.php
@@ -13,4 +13,10 @@
     return pht('English (US)');
   }
 
+  public function getFallbackLocaleCode() {
+    // The default fallback is en_US, explicitly return null here
+    // to avoid a fallback loop
+    return null;
+  }
+
 }