Stop setting the LANG env var (#306)

Previously the buildpack would set the `LANG` env var at build
and run time to the value `C.UTF-8`, to match the official Docker
Python images.

However, the env var should not be needed for modern Python,
and so the official Docker Python images no longer set it:
docker-library/python#887
docker-library/python#895

In addition, the older issues typically only affected other distros
such as alpine, or environments where someone had overridden
the system locale, which doesn't apply to the Heroku base images.

GUS-W-17443565.

Author: edmorley

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Removed
+
+- Stopped setting the `LANG` env var. ([#306](https://github.com/heroku/buildpacks-python/pull/306))
+
 ## [0.20.1] - 2024-12-13
 
 ### Fixed

src/layers/python.rs

@@ -176,14 +176,6 @@ fn generate_layer_env(layer_path: &Path, python_version: &PythonVersion) -> Laye
             )),
         )
         .chainable_insert(Scope::Build, ModificationBehavior::Delimiter, "CPATH", ":")
-        // Ensure Python uses a Unicode locale, to prevent the issues described in:
-        // https://github.com/docker-library/python/pull/570
-        .chainable_insert(
-            Scope::All,
-            ModificationBehavior::Override,
-            "LANG",
-            "C.UTF-8",
-        )
         // We have to set `PKG_CONFIG_PATH` explicitly, since the automatic path set by lifecycle/libcnb
         // is `<layer>/pkgconfig/`, whereas Python's pkgconfig files are at `<layer>/lib/pkgconfig/`.
         .chainable_insert(
@@ -330,7 +322,6 @@ mod tests {
     fn python_layer_env() {
         let mut base_env = Env::new();
         base_env.insert("CPATH", "/base");
-        base_env.insert("LANG", "this-should-be-overridden");
         base_env.insert("PKG_CONFIG_PATH", "/base");
         base_env.insert("PYTHONHOME", "this-should-be-overridden");
         base_env.insert("PYTHONUNBUFFERED", "this-should-be-overridden");
@@ -341,7 +332,6 @@ mod tests {
             utils::environment_as_sorted_vector(&layer_env.apply(Scope::Build, &base_env)),
             [
                 ("CPATH", "/layer-dir/include/python3.11:/base"),
-                ("LANG", "C.UTF-8"),
                 ("PKG_CONFIG_PATH", "/layer-dir/lib/pkgconfig:/base"),
                 ("PYTHONHOME", "/layer-dir"),
                 ("PYTHONUNBUFFERED", "1"),
@@ -352,7 +342,6 @@ mod tests {
             utils::environment_as_sorted_vector(&layer_env.apply(Scope::Launch, &base_env)),
             [
                 ("CPATH", "/base"),
-                ("LANG", "C.UTF-8"),
                 ("PKG_CONFIG_PATH", "/base"),
                 ("PYTHONHOME", "/layer-dir"),
                 ("PYTHONUNBUFFERED", "1"),

tests/pip_test.rs

@@ -40,7 +40,6 @@ fn pip_basic_install_and_cache_reuse() {
                 
                 ## Testing buildpack ##
                 CPATH=/layers/heroku_python/venv/include:/layers/heroku_python/python/include/python3.13:/layers/heroku_python/python/include
-                LANG=C.UTF-8
                 LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib:/layers/heroku_python/pip/lib
                 LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib:/layers/heroku_python/pip/lib
                 PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/layers/heroku_python/pip/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
@@ -84,7 +83,6 @@ fn pip_basic_install_and_cache_reuse() {
         assert_eq!(
             command_output.stdout,
             formatdoc! {"
-                LANG=C.UTF-8
                 LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib
                 PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                 PIP_PYTHON=/layers/heroku_python/venv

tests/poetry_test.rs

@@ -38,7 +38,6 @@ fn poetry_basic_install_and_cache_reuse() {
                 
                 ## Testing buildpack ##
                 CPATH=/layers/heroku_python/venv/include:/layers/heroku_python/python/include/python3.13:/layers/heroku_python/python/include
-                LANG=C.UTF-8
                 LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib:/layers/heroku_python/poetry/lib
                 LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib:/layers/heroku_python/poetry/lib
                 PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/layers/heroku_python/poetry/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
@@ -77,7 +76,6 @@ fn poetry_basic_install_and_cache_reuse() {
         assert_eq!(
             command_output.stdout,
             formatdoc! {"
-                LANG=C.UTF-8
                 LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib
                 PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                 PYTHONHOME=/layers/heroku_python/python