Add ability to filter followed accounts' posts by language (#19095)

app/validators/language_validator.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class LanguageValidator < ActiveModel::EachValidator
+  include LanguagesHelper
+
+  def validate_each(record, attribute, value)
+    record.errors.add(attribute, :invalid) unless valid?(value)
+  end
+
+  private
+
+  def valid?(str)
+    if str.nil?
+      true
+    elsif str.is_a?(Array)
+      str.all? { |x| valid_locale?(x) }
+    else
+      valid_locale?(str)
+    end
+  end
+end