diff --git a/lib/i18n/tasks/data/router/isolating_router.rb b/lib/i18n/tasks/data/router/isolating_router.rb
index f2ca2e5b..be55aaa3 100644
--- a/lib/i18n/tasks/data/router/isolating_router.rb
+++ b/lib/i18n/tasks/data/router/isolating_router.rb
@@ -9,11 +9,12 @@ module Data::Router
     class IsolatingRouter
       include ::I18n::Tasks::KeyPatternMatching
 
-      attr_reader :config_read_patterns, :base_locale
+      attr_reader :config_read_patterns, :base_locale, :locales
 
       def initialize(_adapter, data_config)
         @base_locale = data_config[:base_locale]
         @config_read_patterns = Array.wrap(data_config[:read])
+        @locales = Array.wrap(data_config[:locales] || @base_locale)
       end
 
       # Route keys to destinations
@@ -51,8 +52,9 @@ def route(locale, forest, &block)
       def alternate_path_for(source_path, locale)
         source_path = source_path.dup
 
+        locale_pattern = "{#{locales.join(',')}}"
         config_read_patterns.each do |pattern|
-          regexp = Glob.new(format(pattern, locale: '(*)')).to_regexp
+          regexp = Glob.new(format(pattern, locale: locale_pattern)).to_regexp
           next unless source_path.match?(regexp)
 
           source_path.match(regexp) do |match_data|
diff --git a/spec/isolating_router_spec.rb b/spec/isolating_router_spec.rb
index 42bcf15d..3a10c097 100644
--- a/spec/isolating_router_spec.rb
+++ b/spec/isolating_router_spec.rb
@@ -28,6 +28,7 @@
     I18n::Tasks::Data::FileSystem.new(
       router: 'isolating_router',
       base_locale: 'en',
+      locales: %w[en fr],
       read: ['app/components/*.%{locale}.yml']
     )
   end
@@ -56,7 +57,8 @@
 
   describe 'alternate_path_for(source_path, locale)' do
     let(:read_config_patterns) { ['config/locales/**/*.%{locale}.yml'] }
-    let(:router) { I18n::Tasks::Data::Router::IsolatingRouter.new(nil, { read: read_config_patterns }) }
+    let(:base_locale) { :en }
+    let(:router) { I18n::Tasks::Data::Router::IsolatingRouter.new(nil, { base_locale: base_locale, read: read_config_patterns }) }
 
     context 'when `source_path` matches a pattern of the `read` configuration' do
       it 'changes only the `%{locale}` part of `source_path`' do