feat(frontend): use plain-language validation messages

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`)

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;
+	}
+};