AwsProxyHttpServletRequest#getLocale incorrect for locales with country subtag

[kerkhofsd]

Serverless Java Container version: 1.9.1

Implementations: Spring Boot 2

Framework version: SpringBoot 2.7.10

Frontend service: REST API

Deployment method: Serverless Framework

Scenario

We are relying on Spring's AcceptHeaderLocaleResolver to store the Locale from the Accept-Language header in the LocaleContextHolder.
This AcceptHeaderLocaleResolver uses HttpServletRequest#getLocale() to retrieve the locale from the request. The HttpServletRequest implementation is provided by the aws-serverless-java-container as AwsProxyHttpServletRequest

Expected behavior

AwsProxyHttpServletRequest#getLocale() provides a correct locale, both for locales with (e.g. en-GB) and without (e.g. en) a country tag.

Actual behavior

AwsProxyHttpServletRequest#getLocale() constructs a Locale from the Accept-Language header using the single value Locale constructor.
For values containing a country tag (e.g. en-GB), an incorrect locale is constructed with en-gb as language code and no country code.

Suggested solution

Use Locale#forLanguageTag to construct the locale from the Accept-Language header value.

Additional info

On first sight I would say that AwsHttpApiV2ProxyHttpServletRequest has the same problem.