Language detection refactor (#2099)

* Extract detect_language to separate class

* Use default locale, not just en

* Add spec to confirm that whatlanguage cant identify empty string

* Allow account locale to override default in language detector

* PostStatusService supplies an account to detect language

app/lib/language_detector.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class LanguageDetector
+  attr_reader :text, :account
+
+  def initialize(text, account = nil)
+    @text = text
+    @account = account
+  end
+
+  def to_iso_s
+    WhatLanguage.new(:all).language_iso(text) || default_locale.to_sym
+  end
+
+  private
+
+  def default_locale
+    account&.user&.locale || I18n.default_locale
+  end
+end

app/services/post_status_service.rb

@@ -19,7 +19,7 @@ class PostStatusService < BaseService
                                       sensitive: options[:sensitive],
                                       spoiler_text: options[:spoiler_text] || '',
                                       visibility: options[:visibility],
-                                      language: detect_language(text),
+                                      language: detect_language_for(text, account),
                                       application: options[:application])
 
     attach_media(status, media)
@@ -52,8 +52,8 @@ class PostStatusService < BaseService
     media.update(status_id: status.id)
   end
 
-  def detect_language(text)
-    WhatLanguage.new(:all).language_iso(text) || 'en'
+  def detect_language_for(text, account)
+    LanguageDetector.new(text, account).to_iso_s
   end
 
   def process_mentions_service