diff --git a/frontend/messages/en.json b/frontend/messages/en.json
index d4578c0d..55c39e35 100644
--- a/frontend/messages/en.json
+++ b/frontend/messages/en.json
@@ -1101,5 +1101,15 @@
 	"register_your_first_api": "Register your first API",
 	"create_your_first_api_key": "Create your first API key",
 	"add_your_first_group": "Add your first group",
-	"add_your_first_oidc_client": "Add your first OIDC client"
+	"add_your_first_oidc_client": "Add your first OIDC client",
+	"validation_required": "Required",
+	"validation_min_length": "Enter at least {count} characters",
+	"validation_max_length": "Enter at most {count} characters",
+	"validation_min_number": "Enter {minimum} or more",
+	"validation_max_number": "Enter {maximum} or less",
+	"validation_min_items": "Choose at least {count}",
+	"validation_max_items": "Choose at most {count}",
+	"validation_email": "Enter a valid email address",
+	"validation_url": "Enter a valid URL",
+	"validation_number": "Enter a number"
 }
diff --git a/frontend/src/lib/utils/locale.util.ts b/frontend/src/lib/utils/locale.util.ts
index adc16bb0..7e73411c 100644
--- a/frontend/src/lib/utils/locale.util.ts
+++ b/frontend/src/lib/utils/locale.util.ts
@@ -1,25 +1,23 @@
-import {
-	extractLocaleFromCookie,
-	setLocale as setParaglideLocale,
-	type Locale
-} from '#lib/paraglide/runtime.js';
+import { getLocale, setLocale as setParaglideLocale, type Locale } from '#lib/paraglide/runtime.js';
 import { setDefaultOptions } from 'date-fns';
 import { z } from 'zod/v4';
+import { validationErrorMap } from './validation-error-util';
 
 export async function setLocale(locale: Locale, reload = true) {
 	await setLocaleForLibraries(locale);
 	setParaglideLocale(locale, { reload });
-	document.documentElement.lang = locale;
 }
 
-export async function setLocaleForLibraries(
-	locale: Locale = (extractLocaleFromCookie() as Locale) || 'en'
-) {
+export async function setLocaleForLibraries(locale: Locale = getLocale()) {
+	document.documentElement.lang = locale;
+
 	let dateFnsLocale: string = locale;
 	if (dateFnsLocale === 'en') {
 		dateFnsLocale = 'en-US'; // datefns doesn't have 'en'
 	}
 
+	z.config({ customError: validationErrorMap });
+
 	const [zodResult, dateFnsResult] = await Promise.allSettled([
 		import(`../../../node_modules/zod/v4/locales/${locale}.js`),
 		import(`../../../node_modules/date-fns/locale/${dateFnsLocale}.js`)
diff --git a/frontend/src/lib/utils/validation-error-util.ts b/frontend/src/lib/utils/validation-error-util.ts
new file mode 100644
index 00000000..fd0ecb4f
--- /dev/null
+++ b/frontend/src/lib/utils/validation-error-util.ts
@@ -0,0 +1,43 @@
+import { m } from '#lib/paraglide/messages.js';
+import type { z } from 'zod/v4';
+
+const isEmpty = (input: unknown) => input === undefined || input === null || input === '';
+
+// Plain-language messages for the checks forms use; anything not covered falls back to Zod's own locale
+export const validationErrorMap: z.core.$ZodErrorMap = (issue) => {
+	switch (issue.code) {
+		case 'invalid_type':
+			if (isEmpty(issue.input)) return m.validation_required();
+			if (issue.expected === 'number' || issue.expected === 'int') return m.validation_number();
+			return undefined;
+		case 'too_small': {
+			const count = Number(issue.minimum);
+			if (issue.origin === 'string') {
+				return count <= 1 ? m.validation_required() : m.validation_min_length({ count });
+			}
+			if (issue.origin === 'number' || issue.origin === 'int') {
+				return m.validation_min_number({ minimum: count });
+			}
+			if (issue.origin === 'array' || issue.origin === 'set')
+				return m.validation_min_items({ count });
+			return undefined;
+		}
+		case 'too_big': {
+			const count = Number(issue.maximum);
+			if (issue.origin === 'string') return m.validation_max_length({ count });
+			if (issue.origin === 'number' || issue.origin === 'int') {
+				return m.validation_max_number({ maximum: count });
+			}
+			if (issue.origin === 'array' || issue.origin === 'set')
+				return m.validation_max_items({ count });
+			return undefined;
+		}
+		case 'invalid_format':
+			if (isEmpty(issue.input)) return m.validation_required();
+			if (issue.format === 'email') return m.validation_email();
+			if (issue.format === 'url') return m.validation_url();
+			return undefined;
+		default:
+			return undefined;
+	}
+};
diff --git a/tests/specs/user-signup.spec.ts b/tests/specs/user-signup.spec.ts
index 053495c2..111fafc8 100644
--- a/tests/specs/user-signup.spec.ts
+++ b/tests/specs/user-signup.spec.ts
@@ -208,9 +208,10 @@ test.describe('User Signup', () => {
 
 			await page.goto('/signup');
 
+			await page.getByLabel('Email').fill('not-an-email');
 			await page.getByRole('button', { name: 'Create account' }).click();
 
-			await expect(page.getByText('Invalid email address').first()).toBeVisible();
+			await expect(page.getByText('Enter a valid email address')).toBeVisible();
 		});
 
 		test('Open signup - duplicate email shows error', async ({ page }) => {