diff --git a/CHANGES.md b/CHANGES.md
index f334ca59..2b6a3044 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,9 @@
 ## v0.9.19 (not yet released)
 
+Adds a new configuration setting, `data.external`, for locale data from external dependencies (e.g. gems).
+This locale data is never considered unused, and is never modified by i18n-tasks.
+[#264](https://github.com/glebm/i18n-tasks/issues/264)
+
 Fixes support for calls such as `t @instance_variable, scope: :static_scope` in the non-AST scanner.
 [#1d2c6d0c](https://github.com/glebm/i18n-tasks/commit/1d2c6d0cb7ee20a8db68c52e33ec3c2a382633e6)
 
diff --git a/lib/i18n/tasks/data.rb b/lib/i18n/tasks/data.rb
index c86a1c05..7f20a217 100644
--- a/lib/i18n/tasks/data.rb
+++ b/lib/i18n/tasks/data.rb
@@ -59,6 +59,10 @@ def key_value?(key, locale = base_locale)
       !t(key, locale).nil?
     end
 
+    def external_key?(key, locale = base_locale)
+      data.external(locale)[locale.to_s][key]
+    end
+
     # Normalize all the locale data in the store (by writing to the store).
     #
     # @param [Array<String>] locales locales to normalize. Default: all.
diff --git a/lib/i18n/tasks/data/file_system_base.rb b/lib/i18n/tasks/data/file_system_base.rb
index 9665c869..a8d61e84 100644
--- a/lib/i18n/tasks/data/file_system_base.rb
+++ b/lib/i18n/tasks/data/file_system_base.rb
@@ -32,13 +32,20 @@ def initialize(config = {})
         end
       end
 
-      # get locale tree
+      # @param [String, Symbol] locale
+      # @return [::I18n::Tasks::Data::Siblings]
       def get(locale)
         locale = locale.to_s
-        @trees         ||= {}
-        @trees[locale] ||= Tree::Siblings[locale => {}].merge!(
-          read_locale(locale)
-        )
+        @trees ||= {}
+        @trees[locale] ||= read_locale(locale)
+      end
+
+      # @param [String, Symbol] locale
+      # @return [::I18n::Tasks::Data::Siblings]
+      def external(locale)
+        locale = locale.to_s
+        @external ||= {}
+        @external[locale] ||= read_locale(locale, paths: config[:external])
       end
 
       alias [] get
@@ -151,17 +158,17 @@ def router
 
       protected
 
-      def read_locale(locale)
-        Array(config[:read]).map do |path|
+      def read_locale(locale, paths: config[:read])
+        Array(paths).flat_map do |path|
           Dir.glob format(path, locale: locale)
-        end.reduce(:+).map do |path|
+        end.map do |path|
           [path.freeze, load_file(path) || {}]
         end.map do |path, data|
           filter_nil_keys! path, data
           Data::Tree::Siblings.from_nested_hash(data).tap do |s|
             s.leaves { |x| x.data.update(path: path, locale: locale) }
           end
-        end.reduce(:merge!) || Tree::Siblings.null
+        end.reduce(Tree::Siblings[locale => {}], :merge!)
       end
 
       def filter_nil_keys!(path, data, suffix = [])
diff --git a/lib/i18n/tasks/missing_keys.rb b/lib/i18n/tasks/missing_keys.rb
index 85d63639..44bd3124 100644
--- a/lib/i18n/tasks/missing_keys.rb
+++ b/lib/i18n/tasks/missing_keys.rb
@@ -85,7 +85,7 @@ def equal_values_tree(locale, compare_to = base_locale)
     end
 
     def locale_key_missing?(locale, key)
-      !key_value?(key, locale) && !ignore_key?(key, :missing)
+      !key_value?(key, locale) && !external_key?(key) && !ignore_key?(key, :missing)
     end
 
     # @param [::I18n::Tasks::Data::Tree::Siblings] forest
diff --git a/spec/fixtures/app/views/usages.html.slim b/spec/fixtures/app/views/usages.html.slim
index fdf9f17f..23eb4947 100644
--- a/spec/fixtures/app/views/usages.html.slim
+++ b/spec/fixtures/app/views/usages.html.slim
@@ -1,2 +1,4 @@
 p = t 'used.a'
 b = t 'used.a'
+
+p = t 'external.used'
diff --git a/spec/fixtures/config/i18n-tasks.yml.erb b/spec/fixtures/config/i18n-tasks.yml.erb
index 59774a33..9d5bdc7b 100644
--- a/spec/fixtures/config/i18n-tasks.yml.erb
+++ b/spec/fixtures/config/i18n-tasks.yml.erb
@@ -15,6 +15,8 @@ data:
     - ['devise.*', 'config/locales/devise.%{locale}.yml']
     # default catch-all (same as ['*', 'config/locales/%{locale}.yml'])
     - 'config/locales/%{locale}.yml'
+  external:
+    - 'config/locales/external/%{locale}.yml'
   yaml:
     write:
       line_width: -1
diff --git a/spec/i18n_tasks_spec.rb b/spec/i18n_tasks_spec.rb
index 9d97ba86..5c431c31 100644
--- a/spec/i18n_tasks_spec.rb
+++ b/spec/i18n_tasks_spec.rb
@@ -363,6 +363,10 @@
     fs = fixtures_contents.merge(
       'config/locales/en.yml' => { 'en' => en_data }.to_yaml,
       'config/locales/es.yml' => { 'es' => es_data }.to_yaml,
+      'config/locales/external/en.yml' =>
+          { 'en' => { 'external' => { 'used' => 'EN_TEXT', 'unused' => 'EN_TEXT' } } }.to_yaml,
+      'config/locales/external/es.yml' =>
+          { 'es' => { 'external' => { 'used' => 'ES_TEXT', 'unused' => 'ES_TEXT' } } }.to_yaml,
       'config/locales/old_devise.en.yml' => { 'en' => { 'devise' => { 'a' => 'EN_TEXT' } } }.to_yaml,
       'config/locales/old_devise.es.yml' => { 'es' => { 'devise' => { 'a' => 'ES_TEXT' } } }.to_yaml,
       'config/locales/unused.en.yml' => { 'en' => { 'unused' => { 'file' => 'EN_TEXT' } } }.to_yaml,
diff --git a/templates/config/i18n-tasks.yml b/templates/config/i18n-tasks.yml
index 39e9f59b..a5248346 100644
--- a/templates/config/i18n-tasks.yml
+++ b/templates/config/i18n-tasks.yml
@@ -19,8 +19,6 @@ data:
     # - config/locales/%{locale}.yml
     ## More files:
     # - config/locales/**/*.%{locale}.yml
-    ## Another gem (replace %#= with %=):
-    # - "<%#= %x[bundle show vagrant].chomp %>/templates/locales/%{locale}.yml"
 
   # Locale files to write new keys to, based on a list of key pattern => file rules. Matched from top to bottom:
   # `i18n-tasks normalize -p` will force move the keys according to these rules
@@ -30,6 +28,12 @@ data:
     ## Catch-all default:
     # - config/locales/%{locale}.yml
 
+  # External locale data (e.g. gems).
+  # This data is not considered unused and is never written to.
+  external:
+    ## Example (replace %#= with %=):
+    # - "<%#= %x[bundle show vagrant].chomp %>/templates/locales/%{locale}.yml"
+
   ## Specify the router (see Readme for details). Valid values: conservative_router, pattern_router, or a custom class.
   # router: conservative_router