diff --git a/tests/unit/test.program.js b/tests/unit/test.program.js
index e159aec3e8..15cb07fcbb 100644
--- a/tests/unit/test.program.js
+++ b/tests/unit/test.program.js
@@ -1,7 +1,7 @@
 import path from 'path';
 import fs from 'fs/promises';
 
-import { describe, it } from 'mocha';
+import { after, before, describe, it } from 'mocha';
 import git from 'git-rev-sync';
 import * as sinon from 'sinon';
 import { assert } from 'chai';
@@ -30,6 +30,23 @@ import {
 const { spy } = sinon;
 
 describe('program.Program', () => {
+  let originalLcAll;
+
+  before(() => {
+    // yargs localizes some validation errors based on the environment locale.
+    // Keep tests that assert on these messages deterministic across systems.
+    originalLcAll = process.env.LC_ALL;
+    process.env.LC_ALL = 'en_US.UTF-8';
+  });
+
+  after(() => {
+    if (originalLcAll === undefined) {
+      delete process.env.LC_ALL;
+    } else {
+      process.env.LC_ALL = originalLcAll;
+    }
+  });
+
   function execProgram(program, options = {}) {
     const fakeProcess = createFakeProcess();
     const absolutePackageDir = path.join(