Add extensive controller and model tests

tests/Feature/I18nControllerTest.php

@@ -0,0 +1,13 @@
+<?php
+
+it('returns translations for valid locale', function (): void {
+    $response = $this->getJson('/api/i18n/en');
+
+    $response->assertOk();
+});
+
+it('returns 404 for missing locale', function (): void {
+    $response = $this->getJson('/api/i18n/xx');
+
+    $response->assertStatus(404);
+});