diff --git a/test/test_http_range.rb b/test/test_http_range.rb
new file mode 100644
index 0000000..043a378
--- /dev/null
+++ b/test/test_http_range.rb
@@ -0,0 +1,82 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+class HttpRangeTest < Minitest::Test
+  def test_no_header_returns_nil
+    assert_nil Prism::HttpRange.parse(nil, 100)
+  end
+
+  def test_non_bytes_unit_is_ignored
+    assert_nil Prism::HttpRange.parse('items=0-1', 100)
+  end
+
+  def test_multi_range_is_ignored_not_unsatisfiable
+    assert_nil Prism::HttpRange.parse('bytes=0-10,20-30', 100)
+  end
+
+  def test_simple_start_end
+    result = Prism::HttpRange.parse('bytes=0-9', 100)
+    assert_equal 0, result.start
+    assert_equal 9, result.finish
+    assert_equal 10, result.length
+  end
+
+  def test_open_ended_range_clamps_to_file_size
+    result = Prism::HttpRange.parse('bytes=90-', 100)
+    assert_equal 90, result.start
+    assert_equal 99, result.finish
+    assert_equal 10, result.length
+  end
+
+  def test_end_beyond_file_size_is_clamped_not_rejected
+    result = Prism::HttpRange.parse('bytes=0-999', 100)
+    assert_equal 99, result.finish
+  end
+
+  def test_suffix_range_returns_last_n_bytes
+    result = Prism::HttpRange.parse('bytes=-10', 100)
+    assert_equal 90, result.start
+    assert_equal 99, result.finish
+    assert_equal 10, result.length
+  end
+
+  def test_suffix_range_larger_than_file_clamps_to_whole_file
+    result = Prism::HttpRange.parse('bytes=-1000', 100)
+    assert_equal 0, result.start
+    assert_equal 99, result.finish
+  end
+
+  def test_start_beyond_file_size_is_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=100-200', 100)
+  end
+
+  def test_start_after_end_is_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=50-10', 100)
+  end
+
+  def test_empty_start_and_end_is_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=-', 100)
+  end
+
+  def test_garbage_spec_is_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=abc-def', 100)
+  end
+
+  def test_zero_length_suffix_is_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=-0', 100)
+  end
+
+  def test_empty_file_is_always_unsatisfiable
+    assert_equal :unsatisfiable, Prism::HttpRange.parse('bytes=0-0', 0)
+  end
+
+  def test_read_slice_returns_only_the_requested_bytes
+    Dir.mktmpdir do |dir|
+      path = File.join(dir, 'data.bin')
+      File.binwrite(path, '0123456789')
+      slice = Prism::HttpRange.parse('bytes=2-5', 10)
+      assert_equal '2345', Prism::HttpRange.read_slice(path, slice)
+    end
+  end
+end
diff --git a/test/test_i18n.rb b/test/test_i18n.rb
new file mode 100644
index 0000000..cec3fa9
--- /dev/null
+++ b/test/test_i18n.rb
@@ -0,0 +1,90 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+class I18nModuleTest < Minitest::Test
+  def setup
+    Prism::I18n.reset!
+    Prism::I18n.store_translations(:en, 'hello' => 'Hello')
+    Prism::I18n.store_translations(:pl, 'hello' => 'Czesc')
+  end
+
+  def teardown
+    Prism::I18n.reset!
+  end
+
+  def test_default_locale_is_en_out_of_the_box
+    assert_equal :en, Prism::I18n.default_locale
+  end
+
+  def test_locale_reads_and_writes_current_locale
+    assert_equal :en, Prism::I18n.locale
+    Prism::I18n.locale = :pl
+    assert_equal :pl, Prism::I18n.locale
+  end
+
+  def test_t_and_l_are_aliases
+    assert_equal Prism::I18n.translate('hello', locale: :en), Prism::I18n.t('hello', locale: :en)
+    time = Time.utc(2026, 1, 1)
+    assert_equal Prism::I18n.localize(time, locale: :en), Prism::I18n.l(time, locale: :en)
+  end
+
+  def test_translate_uses_current_locale_by_default
+    Prism::I18n.locale = :pl
+    assert_equal 'Czesc', Prism::I18n.t('hello')
+  end
+
+  def test_with_locale_is_scoped_and_restores_previous_value
+    Prism::I18n.locale = :en
+    Prism::I18n.with_locale(:pl) do
+      assert_equal 'Czesc', Prism::I18n.t('hello')
+    end
+    assert_equal :en, Prism::I18n.locale
+  end
+
+  def test_with_locale_nests_correctly
+    Prism::I18n.with_locale(:en) do
+      Prism::I18n.with_locale(:pl) do
+        assert_equal :pl, Prism::I18n.locale
+      end
+      assert_equal :en, Prism::I18n.locale
+    end
+  end
+
+  def test_available_locales_reflects_stored_translations
+    assert_equal [:en, :pl], Prism::I18n.available_locales.sort
+  end
+
+  def test_load_translations_reads_every_json_file_under_load_path
+    Dir.mktmpdir do |dir|
+      File.write(File.join(dir, 'fr.json'), '{"hello": "Bonjour"}')
+      Prism::I18n.load_path << dir
+      Prism::I18n.load_translations!
+      assert_equal 'Bonjour', Prism::I18n.t('hello', locale: :fr)
+    end
+  end
+
+  def test_best_locale_from_accept_picks_exact_match_by_q_value
+    header = 'fr;q=0.5, pl;q=0.9, en;q=0.1'
+    assert_equal :pl, Prism::I18n.best_locale_from_accept(header)
+  end
+
+  def test_best_locale_from_accept_matches_primary_subtag
+    assert_equal :en, Prism::I18n.best_locale_from_accept('en-US,en;q=0.8')
+  end
+
+  def test_best_locale_from_accept_returns_nil_when_nothing_available
+    assert_nil Prism::I18n.best_locale_from_accept('de,es;q=0.5')
+  end
+
+  def test_best_locale_from_accept_returns_nil_for_blank_header
+    assert_nil Prism::I18n.best_locale_from_accept('')
+    assert_nil Prism::I18n.best_locale_from_accept(nil)
+  end
+
+  def test_changing_default_locale_invalidates_cached_translator
+    assert_equal 'Hello', Prism::I18n.t('hello', locale: :en)
+    Prism::I18n.default_locale = :pl
+    assert_equal 'Czesc', Prism::I18n.t('hello')
+  end
+end