Use Rails to translate email subject

And translate the subject with the user's locale.
If the subject doesn't need parameters, you don't need to supply it.
Rails finds the translation automatically.

app/mailers/payment_mailer.rb

@@ -6,11 +6,9 @@ class PaymentMailer < ApplicationMailer
   def authorize_payment(payment)
     @payment = payment
     @order = @payment.order
-    subject = I18n.t("payment_mailer.authorize_payment.subject",
-                     distributor: @order.distributor.name)
     I18n.with_locale valid_locale(@order.user) do
       mail(to: @order.email,
-           subject:,
+           subject: default_i18n_subject(distributor: @order.distributor.name),
            reply_to: @order.distributor.contact.email)
     end
   end
@@ -18,24 +16,19 @@ class PaymentMailer < ApplicationMailer
   def authorization_required(payment)
     @order = payment.order
     shop_owner = @order.distributor.owner
-    subject = I18n.t("payment_mailer.authorization_required.subject",
-                     order: @order)
     I18n.with_locale valid_locale(shop_owner) do
-      mail(to: shop_owner.email,
-           subject:,
-           reply_to: @order.email)
+      mail(to: shop_owner.email, reply_to: @order.email)
     end
   end
 
   def refund_available(payment, taler_order_status_url)
     @order = payment.order
+    @shop = @order.distributor.name
     @taler_order_status_url = taler_order_status_url
 
-    subject = I18n.t("payment_mailer.refund_available.subject",
-                     order: @order)
     I18n.with_locale valid_locale(@order.user) do
       mail(to: @order.email,
-           subject:,
+           subject: default_i18n_subject(shop: @shop),
            reply_to: @order.email)
     end
   end

app/views/payment_mailer/refund_available.html.haml

@@ -1,2 +1,2 @@
-%p= t(".message", order_number: @order.number)
+%p= t(".message", shop: @shop)
 %p= link_to @taler_order_status_url, @taler_order_status_url

config/locales/en.yml

@@ -466,8 +466,10 @@ en:
       subject: "A payment requires authorization from the customer"
       message: "A payment for order %{order_number} requires additional authorization from the customer. The customer has been notified via email and the payment will appear as pending until it is authorized."
     refund_available:
-      subject: "Refund available"
-      message: "Your payment for order %{order_number} is being refunded. Claim your refund following the link below."
+      subject: "Refund from %{shop}"
+      message: |
+        Your payment to %{shop} is being refunded.
+        Accept your refund following the link below.
   producer_mailer:
     order_cycle:
       subject: "Order cycle report for %{producer}"

spec/mailers/payment_mailer_spec.rb

@@ -2,7 +2,6 @@
 
 RSpec.describe PaymentMailer do
   describe '#payment_mailer' do
-    let(:enterprise) { create(:enterprise) }
     let(:payment_method) {
       create(:payment_method, distributors: [order.distributor])
     }
@@ -15,7 +14,8 @@ RSpec.describe PaymentMailer do
       subject(:email) { described_class.authorize_payment(payment) }
 
       it "includes the distributor's name in the subject" do
-        expect(email.subject).to include("authorize your payment to #{order.distributor.name}")
+        order.distributor.name = "Fennel Farmer"
+        expect(email.subject).to include("authorize your payment to Fennel Farmer")
       end
 
       it "sets a reply-to of the customer email" do
@@ -44,11 +44,12 @@ RSpec.describe PaymentMailer do
   describe "#refund_available" do
     it "tells the user to accept a refund" do
       payment = create(:payment)
+      payment.order.distributor = create(:enterprise, name: "Carrot Castle")
       link = "https://taler.example.com/order/1"
       mail = PaymentMailer.refund_available(payment, link)
 
-      expect(mail.subject).to eq "Refund available"
-      expect(mail.body).to match "Claim your refund following the link below."
+      expect(mail.subject).to eq "Refund from Carrot Castle"
+      expect(mail.body).to match "Your payment to Carrot Castle is being refunded."
       expect(mail.body).to match link
     end
   end