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