diff --git a/src/web/l10n.mjs b/src/web/l10n.mjs
index f286ff0..954ea5a 100644
--- a/src/web/l10n.mjs
+++ b/src/web/l10n.mjs
@@ -33,7 +33,7 @@ export class L10n {
   }
 
   constructor(language) {
-    this.language = language || "en-US";
+    this.language = language || "en";
     this.ready = this.load().then(() => true);
   }
 
@@ -42,7 +42,7 @@ export class L10n {
       const [locale, fallbackLocale, defaultLocale] = await Promise.all([
         L10n.loadLocale(this.language),
         L10n.loadLocale(this.language.split("-")[0]),
-        L10n.loadLocale("en-US"),
+        L10n.loadLocale("en"),
       ]);
       this.locale = locale;
       this.fallbackLocale = fallbackLocale;
diff --git a/tests/fixtures/locales/en.json b/tests/fixtures/locales/en.json
new file mode 100644
index 0000000..36373c4
--- /dev/null
+++ b/tests/fixtures/locales/en.json
@@ -0,0 +1,7 @@
+{
+  "effectiveMessage": "Message with effective content",
+  "blankMessage": "Blank message",
+  "messageWithPlaceholders": "Message with placeholders: ${one}, ${two}",
+  "missingFallbackMessage": "Message defined in fallback locales",
+  "missingMessage": "Message not defined in non-default locales"
+}
diff --git a/tests/fixtures/locales/ja-JP.json b/tests/fixtures/locales/ja-JP.json
new file mode 100644
index 0000000..b790273
--- /dev/null
+++ b/tests/fixtures/locales/ja-JP.json
@@ -0,0 +1,7 @@
+{
+  "effectiveMessage": "JP：意味ある内容を含むメッセージ",
+  "blankMessage": "",
+  "messageWithPlaceholders": "JP：プレースホルダーを含むメッセージ：${one}, ${two}, ${three}",
+  "_missingFallbackMessage": "JP：フォールバック先で定義されているメッセージ",
+  "_missingMessage": "JP：未定義のメッセージ"
+}
diff --git a/tests/fixtures/locales/ja.json b/tests/fixtures/locales/ja.json
new file mode 100644
index 0000000..a5f9cbc
--- /dev/null
+++ b/tests/fixtures/locales/ja.json
@@ -0,0 +1,7 @@
+{
+  "effectiveMessage": "意味ある内容を含むメッセージ",
+  "blankMessage": "空のメッセージ",
+  "messageWithPlaceholders": "プレースホルダーを含むメッセージ：${one}, ${two}",
+  "missingFallbackMessage": "フォールバック先で定義されているメッセージ",
+  "_missingMessage": "未定義のメッセージ"
+}
diff --git a/tests/unit/test-l10n.mjs b/tests/unit/test-l10n.mjs
new file mode 100644
index 0000000..4fb0bb6
--- /dev/null
+++ b/tests/unit/test-l10n.mjs
@@ -0,0 +1,50 @@
+/*
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+*/
+'use strict';
+
+import "./l10n.mjs";
+import { L10n } from "../../src/web/l10n.mjs";
+import { assert } from "tiny-esm-test-runner";
+const { is } = assert;
+
+let l10n;
+
+export async function setUp() {
+  L10n.clearCache();
+  L10n.baseUrl = (new URL(`${import.meta.url}/../../fixtures/`)).toString();
+  l10n = new L10n("ja-JP");
+  await l10n.ready;
+}
+
+test_get.parameters = {
+  effective: {
+    key: "effectiveMessage",
+    expected: "JP：意味ある内容を含むメッセージ",
+  },
+  blank: {
+    key: "blankMessage",
+    expected: "",
+  },
+  withPlaceholders: {
+    key: "messageWithPlaceholders",
+    params: {
+      one: "One",
+      two: "Two",
+    },
+    expected: "JP：プレースホルダーを含むメッセージ：One, Two, ${three}",
+  },
+  fallbackToGeneralLocale: {
+    key: "missingFallbackMessage",
+    expected: "フォールバック先で定義されているメッセージ",
+  },
+  fallbackToDefaultLocale: {
+    key: "missingMessage",
+    expected: "Message not defined in non-default locales",
+  },
+};
+export function test_get({ key, params, expected }) {
+  is(expected, l10n.get(key, params || null));
+}