diff --git a/README.md b/README.md
index e5b9017..70c2bc3 100644
--- a/README.md
+++ b/README.md
@@ -6,4 +6,4 @@
 - **string_utils** — String manipulation utilities: slugify, truncate, case conversion (snake_case, camelCase, PascalCase), and more.
 - **math_utils** — Math utilities: clamp, lerp, safe_divide, remap, and more.
 - **collection_utils** — Collection utilities: chunk, flatten, unique_by, group_by, partition, compact.
-- **constants** — Package-level constants (DEFAULT_HELLO_TARGET, DEFAULT_FAREWELL).
+- **constants** — Package-level constants (DEFAULT_HELLO_TARGET, DEFAULT_FAREWELL, DEFAULT_LANGUAGE).
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 68d359e..436b586 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file.
 
+## [v006] - 2026-05-14
+
+### Added
+- `DEFAULT_LANGUAGE: str = "en"` constant to `constants` module (BL-020)
+- Test functions `test_default_language_value` and `test_default_language_type` in `test_constants.py`
+
 ## [v005] - 2026-05-13
 
 ### Added
diff --git a/src/test_target_2/constants.py b/src/test_target_2/constants.py
index 75bc36f..c123c6f 100644
--- a/src/test_target_2/constants.py
+++ b/src/test_target_2/constants.py
@@ -2,3 +2,4 @@
 
 DEFAULT_HELLO_TARGET: str = "world"
 DEFAULT_FAREWELL: str = "goodbye"
+DEFAULT_LANGUAGE: str = "en"
diff --git a/tests/test_constants.py b/tests/test_constants.py
index a6599dd..e4c229c 100644
--- a/tests/test_constants.py
+++ b/tests/test_constants.py
@@ -25,3 +25,13 @@ def test_default_farewell_value() -> None:
 
 def test_default_farewell_type() -> None:
     assert isinstance(constants.DEFAULT_FAREWELL, str)
+
+
+def test_default_language_value() -> None:
+    """DEFAULT_LANGUAGE has expected value."""
+    assert constants.DEFAULT_LANGUAGE == "en"
+
+
+def test_default_language_type() -> None:
+    """DEFAULT_LANGUAGE is string type."""
+    assert isinstance(constants.DEFAULT_LANGUAGE, str)