tpl/lang: Warn when using reserved translation keys

[Dobefu]

When using a translation key that is reserved in the go-i18n package, Hugo will now log an error instead of silently failing.

Fixes #14061

Please note:

• I've looked into adding a test case, but there does not appear to be any tests for the Translate method. If wanted, I can add some. But perhaps it won't be needed for a minor change like this. Up to the reviewer(s) 🙂.
• Since translation keys may be multiple levels deep, I did not use SplitN with a count of 1.
• I'm intentionally not bounds checking, since an empty string will create an empty slice and won't trigger a runtime exception.

[CLAassistant]

All committers have signed the CLA.

[bep]

Thanks, but this should now be covered in 559a029