diff --git a/tests/test_i18n.py b/tests/test_i18n.py
new file mode 100644
index 00000000..dce0fb9d
--- /dev/null
+++ b/tests/test_i18n.py
@@ -0,0 +1,208 @@
+"""Tests for the i18n internationalization module."""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+import strix.i18n as mod
+from strix.config.settings import Settings
+from strix.i18n import (
+    SUPPORTED_LANGUAGES,
+    _detect_language,
+    _load_locale,
+    _normalize_lang,
+    get_language,
+    get_language_directive,
+    set_language,
+    t,
+)
+
+
+@pytest.fixture(autouse=True)
+def _reset_i18n_state():
+    """Reset module-level state between tests."""
+    mod._language = None
+    mod._locales.clear()
+    yield
+    mod._language = None
+    mod._locales.clear()
+
+
+class TestSetLanguage:
+    def test_set_language_spanish(self):
+        set_language("es")
+        assert get_language() == "es"
+
+    def test_set_language_english(self):
+        set_language("en")
+        assert get_language() == "en"
+
+    def test_set_language_normalizes(self):
+        set_language("ES")
+        assert get_language() == "es"
+
+    def test_set_language_none_resets(self):
+        set_language("es")
+        set_language(None)
+        # After reset, should fall back to detection
+        assert get_language() == "en"  # default
+
+    def test_set_language_unsupported_falls_back(self):
+        set_language("fr")
+        assert get_language() == "en"
+
+
+class TestNormalizeLang:
+    def test_normalize_lowercase(self):
+        assert _normalize_lang("es") == "es"
+
+    def test_normalize_uppercase(self):
+        assert _normalize_lang("ES") == "es"
+
+    def test_normalize_strips(self):
+        assert _normalize_lang("  es  ") == "es"
+
+    def test_normalize_unsupported(self):
+        assert _normalize_lang("fr") == "en"
+
+    def test_normalize_empty(self):
+        assert _normalize_lang("") == "en"
+
+
+class TestDetectLanguage:
+    def test_default_is_english(self):
+        assert _detect_language() == "en"
+
+    def test_explicit_set_takes_priority(self):
+        mod._language = "es"
+        assert _detect_language() == "es"
+
+    def test_env_var_detected(self):
+        with patch.dict(os.environ, {"STRIX_LANGUAGE": "es"}):
+            assert _detect_language() == "es"
+
+    def test_env_var_overrides_config(self):
+        with patch.dict(os.environ, {"STRIX_LANGUAGE": "es"}):
+            assert _detect_language() == "es"
+
+    def test_system_locale_detected(self):
+        with patch.dict(os.environ, {"LANG": "es_ES.UTF-8"}, clear=False):
+            mod._language = None
+            assert _detect_language() == "es"
+
+
+class TestLoadLocale:
+    def test_load_english(self):
+        locale = _load_locale("en")
+        assert "cli.description" in locale
+        assert isinstance(locale, dict)
+
+    def test_load_spanish(self):
+        locale = _load_locale("es")
+        assert "cli.description" in locale
+
+    def test_load_missing_returns_empty(self):
+        locale = _load_locale("nonexistent")
+        assert locale == {}
+
+    def test_cached_after_first_load(self):
+        _load_locale("en")
+        assert "en" in mod._locales
+
+
+class TestTranslationFunction:
+    def test_t_returns_english_by_default(self):
+        result = t("cli.description")
+        assert "Strix" in result
+        assert "Penetration" in result
+
+    def test_t_returns_spanish_when_set(self):
+        set_language("es")
+        result = t("cli.description")
+        assert "Penetración" in result
+
+    def test_t_falls_back_to_english_for_missing_key(self):
+        set_language("es")
+        # Use a key that exists in en but not es
+        result = t("cli.description")
+        # Should still return something (English fallback)
+        assert result != "cli.description"
+
+    def test_t_returns_key_for_missing(self):
+        result = t("nonexistent.key.xyz")
+        assert result == "nonexistent.key.xyz"
+
+    def test_t_interpolates_placeholders(self):
+        result = t("cli.scan_started", target="example.com")
+        assert "example.com" in result
+
+    def test_t_interpolates_spanish(self):
+        set_language("es")
+        result = t("cli.scan_started", target="example.com")
+        assert "example.com" in result
+        assert "escaneo" in result.lower()
+
+    def test_t_handles_missing_placeholder_gracefully(self):
+        # Should not raise, just return with missing placeholder
+        result = t("cli.scan_started")  # target not provided
+        assert isinstance(result, str)
+
+
+class TestLanguageDirective:
+    def test_english_returns_empty(self):
+        set_language("en")
+        directive = get_language_directive()
+        assert directive == ""
+
+    def test_spanish_returns_directive(self):
+        set_language("es")
+        directive = get_language_directive()
+        assert "Spanish" in directive
+        assert "CVE" in directive  # preservation rule
+        assert "CWE" in directive
+
+    def test_directive_preserves_identifiers(self):
+        set_language("es")
+        directive = get_language_directive()
+        assert "CVE identifiers" in directive
+        assert "CWE identifiers" in directive
+        assert "CVSS scores" in directive
+        assert "Source code" in directive
+        assert "Shell commands" in directive
+
+
+class TestLocaleKeyConsistency:
+    def test_all_en_keys_exist_in_es(self):
+        en = _load_locale("en")
+        es = _load_locale("es")
+        missing = set(en.keys()) - set(es.keys())
+        assert missing == set(), f"Missing Spanish keys: {missing}"
+
+    def test_locale_files_are_valid_json(self):
+        locales_dir = Path(__file__).parent.parent / "strix" / "locales"
+        for json_file in locales_dir.glob("*.json"):
+            data = json.loads(json_file.read_text(encoding="utf-8"))
+            assert isinstance(data, dict), f"{json_file.name} is not a dict"
+
+    def test_supported_languages_match_files(self):
+        locales_dir = Path(__file__).parent.parent / "strix" / "locales"
+        for lang in SUPPORTED_LANGUAGES:
+            locale_file = locales_dir / f"{lang}.json"
+            assert locale_file.exists(), f"Missing locale file: {locale_file}"
+
+
+class TestSettingsLanguageField:
+    def test_settings_has_language_field(self):
+        s = Settings()
+        assert hasattr(s, "language")
+        assert s.language == "en"
+
+    def test_settings_language_from_env(self):
+        with patch.dict(os.environ, {"STRIX_LANGUAGE": "es"}):
+            s = Settings()
+            assert s.language == "es"