fix(client): fallback page lang to site lang (close #1365)

[vordimous]

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• [x] Bug fix
• [ ] New feature
• [ ] Documentation update
• [ ] Other

Description

Some pages were rendered with the wrong locale lang set. I have updated the resolver to look at the page data and the siteLocaleData. Additionally, I have set the default to be the same as the other Locale classes: en-US. This should ensure all pages have the correct locale lang set

fixes https://github.com/vuepress/vuepress-next/issues/1365

[Mister-Hope]

LGTM

[meteorlxy]

Thanks for your contributions! ❤️