Leave unknown language as nil if account is remote (#8861)

* Force use language detector if account is remote

* Set unknown remote toot's language as nil

app/lib/language_detector.rb

@@ -12,6 +12,7 @@ class LanguageDetector
   def detect(text, account)
     input_text = prepare_text(text)
     return if input_text.blank?
+
     detect_language_code(input_text) || default_locale(account)
   end
 
@@ -33,6 +34,7 @@ class LanguageDetector
 
   def detect_language_code(text)
     return if unreliable_input?(text)
+
     result = @identifier.find_language(text)
     iso6391(result.language.to_s).to_sym if result.reliable?
   end
@@ -75,6 +77,6 @@ class LanguageDetector
   end
 
   def default_locale(account)
-    account.user_locale&.to_sym || I18n.default_locale
+    return account.user_locale&.to_sym || I18n.default_locale if account.local?
   end
 end

spec/lib/language_detector_spec.rb

@@ -42,6 +42,7 @@ describe LanguageDetector do
 
   describe 'detect' do
     let(:account_without_user_locale) { Fabricate(:user, locale: nil).account }
+    let(:account_remote) { Fabricate(:account, domain: 'joinmastodon.org') }
 
     it 'detects english language for basic strings' do
       strings = [
@@ -104,6 +105,15 @@ describe LanguageDetector do
         end
       end
 
+      describe 'remote user' do
+        it 'nil for foreign user when language is not present' do
+          string = '안녕하세요'
+          result = described_class.instance.detect(string, account_remote)
+
+          expect(result).to eq nil
+        end
+      end
+
       describe 'with a non-`en` default locale' do
         around(:each) do |example|
           before = I18n.default_locale