Checking if translation exists

[chemicstry]

Is your feature request related to a problem? Please describe.

Currently when translation is not available, the return value is just the translation key, which makes checking for translation existence hard.

Describe the solution you'd like

It would be nice to have an option, which would return null when translation is not found instead of translation key.

For example:

// Without specifying options, keyAsDefault is true, which is the current behavior
const t = useTranslations('Error', { keyAsDefault: false });

const error: string = t('myError')!; // I know this must exist
const errorHelp: string | null = t('myErrorHelpText'); // Optional help text, which might not be available

The suggested API is not ideal, maybe someone has a more convenient idea.

Describe alternatives you've considered

The only alternative I could think of is checking if returned string matches key, which is very verbose:

const t = useTranslations('Error');

const error: string = t('myError');
const errorHelp: string | null = t('myErrorHelpText') === 'Error.myErrorHelpText' ? null : t('myErrorHelpText');

[amannn]

This mostly looks like a duplicate of #88.

Note however that the message fallback can be customized, that should likely work for your use case: https://next-intl-docs.vercel.app/docs/configuration#error-handling