docs: Clarify which characters can be used in ICU variables and select values (#967)

Author: amannn

Diff for: docs/pages/docs/usage/messages.mdx

@@ -167,6 +167,12 @@ Dynamic values can be inserted into messages by using curly braces:
 t('message', {name: 'Jane'}); // "Hello Jane!"
 ```
 
+<details>
+<summary>Which characters are supported for value names?</summary>
+
+Value names are required to be alphanumeric and can contain underscores. All other characters, including dashes, are not supported.
+</details>
+
 ### Cardinal pluralization
 
 To express the pluralization of a given number of items, the `plural` argument can be used:
@@ -249,6 +255,23 @@ t('message', {gender: 'female'}); // "She is online."
 
 **Note**: The `other` case is required and will be used when none of the specific values match.
 
+<details>
+<summary>Which characters are supported for select values?</summary>
+
+Values are required to be alphanumeric and can contain underscores. All other characters, including dashes, are not supported.
+
+Therefore, e.g. when you're mapping a locale to a human readable string, you should map the dash to an underscore first:
+
+```tsx filename="en.json"
+"label": "{locale, select, en_BR {British English} en_US {American English} other {Unknown}"
+```
+
+```tsx
+const locale = 'en-BR';
+t('message', {locale: locale.replaceAll('-', '_')};
+```
+</details>
+
 ### Escaping
 
 Since curly braces are used for [interpolating dynamic values](/docs/usage/messages#interpolation-of-dynamic-values), you can escape them with the `'` marker to use the actual symbol in messages:

Diff for: packages/use-intl/test/core/createTranslator.test.tsx

@@ -49,6 +49,30 @@ it('handles formatting errors', () => {
   expect(result).toBe('price');
 });
 
+it('supports alphanumeric value names', () => {
+  const t = createTranslator({
+    locale: 'en',
+    messages: {label: '{val_u3}'}
+  });
+
+  const result = t('label', {val_u3: 'Hello'});
+  expect(result).toBe('Hello');
+});
+
+it('throws an error for non-alphanumeric value names', () => {
+  const onError = vi.fn();
+
+  const t = createTranslator({
+    locale: 'en',
+    messages: {label: '{val-u3}'},
+    onError
+  });
+
+  t('label', {'val-u3': 'Hello'});
+  const error: IntlError = onError.mock.calls[0][0];
+  expect(error.code).toBe('INVALID_MESSAGE');
+});
+
 describe('t.rich', () => {
   it('can translate a message to a ReactNode', () => {
     const t = createTranslator({