EEx.function_from_file did it! I wasn’t quite understanding what you were saying initially. Some test code for anyone else who lands here later:

defmodule Notification.EmailImpl.WelcomeEmailTemplate do
  @moduledoc false

  alias Notification.EmailImpl.EmailTemplateUtil
  import Notification.Gettext
  require EEx

  @external_resource template = Path.join([__DIR__, "template/welcome_template_email.html.eex"])

  EEx.function_from_file(:defp, :template_as_function, template, [:vars])

  def render(%{starts_on: _starts_on, ends_on: _ends_on} = variables) do
    vars = variables
    |> Map.update!(:starts_on, fn date -> EmailTemplateUtil.to_localised_date_string(date) end)
    |> Map.update!(:ends_on, fn date -> EmailTemplateUtil.to_localised_date_string(date) end)
    |> EmailTemplateUtil.compute_values()

    Gettext.with_locale("es", fn -> template_as_function(vars) end)
  end
end

and in the template:

<%= gettext("You have been enrolled in the following tutorial, %{name}, starting on %{starts_on}.", name: vars.name, starts_on: vars.starts_on) %>

Thanks LostKobrakai!