bcp47_language_tag doesn't fail on some non-BCP47 tags #1221

• I have looked at the documentation here first?
• I have looked at the examples provided that may showcase my question here?

Package version eg. v9, v10:

v10

Issue, Question or Enhancement:

When using bcp47_language_tag for validation, some non-BCP47 tags such as "eng" or "en_US" are passing as valid.

‎isBCP47LanguageTag() uses golang.org/x/text/language's Parse and its documentation says:

[snip]
It accepts tags in the BCP 47 format and extensions to this standard defined in https://www.unicode.org/reports/tr35/#Unicode_Language_and_Locale_Identifiers.

Code sample, to showcase or reproduce:

I expect both of these to fail, but they don't:

package main

import (
	"fmt"
	"github.com/go-playground/validator/v10"
)

func main() {
	validate := validator.New()

	err := validate.Var("en_US", "bcp47_language_tag")
	if err != nil {
		fmt.Println(err.Error())
		return
	}

	err = validate.Var("eng", "bcp47_language_tag")
	if err != nil {
		fmt.Println(err.Error())
		return
	}
}