diff --git a/src/internationalization/PhutilLocale.php b/src/internationalization/PhutilLocale.php
index 3ef2524e..5ed03a92 100644
--- a/src/internationalization/PhutilLocale.php
+++ b/src/internationalization/PhutilLocale.php
@@ -30,11 +30,14 @@ abstract class PhutilLocale extends Phobject {
    * 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/PhutilPortugueseBrazilLocale.php b/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
index 2c1ffbea..259ed541 100644
--- a/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
+++ b/src/internationalization/locales/PhutilPortugueseBrazilLocale.php
@@ -13,4 +13,13 @@ final class PhutilPortugueseBrazilLocale extends PhutilLocale {
     return pht('Portuguese (Brazil)');
   }
 
+  public function getFallbackLocaleCode() {
+    // 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 this direction
+    // than the other one
+    return 'pt_PT';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilPortuguesePortugalLocale.php b/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
index f893b77f..f5a9eb71 100644
--- a/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
+++ b/src/internationalization/locales/PhutilPortuguesePortugalLocale.php
@@ -13,4 +13,14 @@ final class PhutilPortuguesePortugalLocale extends PhutilLocale {
     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
index 6b0c709b..4f763323 100644
--- a/src/internationalization/locales/PhutilSimplifiedChineseLocale.php
+++ b/src/internationalization/locales/PhutilSimplifiedChineseLocale.php
@@ -13,4 +13,13 @@ final class PhutilSimplifiedChineseLocale extends PhutilLocale {
     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/PhutilTraditionalChineseLocale.php b/src/internationalization/locales/PhutilTraditionalChineseLocale.php
index f8879506..19ca3edb 100644
--- a/src/internationalization/locales/PhutilTraditionalChineseLocale.php
+++ b/src/internationalization/locales/PhutilTraditionalChineseLocale.php
@@ -13,4 +13,8 @@ final class PhutilTraditionalChineseLocale extends PhutilLocale {
     return pht('Chinese (Traditional)');
   }
 
+  public function getFallbackLocaleCode() {
+    return 'zh_Hans';
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilUSEnglishLocale.php b/src/internationalization/locales/PhutilUSEnglishLocale.php
index ef1633ed..1bd7430c 100644
--- a/src/internationalization/locales/PhutilUSEnglishLocale.php
+++ b/src/internationalization/locales/PhutilUSEnglishLocale.php
@@ -13,4 +13,10 @@ final class PhutilUSEnglishLocale extends PhutilLocale {
     return pht('English (US)');
   }
 
+  public function getFallbackLocaleCode() {
+    // The default fallback is en_US, explicitly return null here
+    // to avoid a fallback loop
+    return null;
+  }
+
 }