What language do you write tests in?

To test Ruby applications, Ruby is usually used. The main frameworks are RSpec and Minitest.

RSpec is more expressive and declarative, suitable for unit and integration tests:

RSpec.describe User do
  it "validates presence of email" do
    user = User.new(email: nil)
    expect(user).not_to be_valid
  end
end

Minitest is built into Ruby, simpler and faster, often used for unit tests:

require 'minitest/autorun'

class UserTest < Minitest::Test
  def test_email_presence
    user = User.new(email: nil)
    refute user.valid?
  end
end

For automating UI tests with Ruby, Capybara is often used together with RSpec.

Thus, tests are written in Ruby using specialized libraries and frameworks that integrate with the codebase.