[gen-l10n] support for unofficial locales

### Use case

I want to be able to use gen-l10n to generate localizations for locales that are not in the official list.
If this is supported I can use it to
- Support different dialects we use in the Netherlands that didn't make the [official list](https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry)
- Use it to switch between more formal and informal texts, depending on the customer using the app

### Proposal

Don't validate locales against the [official registry](https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry) upon generation the localization files. Not sure why the decision was ever made or what the impact may be when we get rid of the check.

An explicit flag to bypass may be worth it so that the check stays in place unless one explicitly disabled.