Add lang attribute to HTML pages

[somiljain2006]

Add a lang attribute to the root element for Jenkins UI pages so the default document language is programmatically determinable.

Fixes #16649

Testing done

Screenshots (UI changes only)

Before

After

Proposed changelog entries

Add a lang attribute to Jenkins HTML pages.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/core-pr-reviewers

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[MarkEWaite]

I think this needs more discussion in the user experience SIG before any implementation. I'd want to hear from people with more experience with accessibility before implementing a change from a single issue report from a first time reporter. The issue report has no additional comments or any hints that there is a real benefit to the user by complying with their company usability guidelines.

[daniel-beck]

If I navigate to https://weekly.ci.jenkins.io/ using the Accept-Language zh-CN (Simplified Chinese), the entire UI is in English. So Jenkins would lie to the browser with this change.

I doubt it's better to provide an incorrect programmatic declaration of language, than none at all.

[somiljain2006]

The request2.getLocale() reflects the requested locale, but not necessarily the language Jenkins renders the UI in, so is there an existing API in Jenkins core or Stapler that exposes the resolved UI language for a page, rather than just the requested locale?

[daniel-beck]

so is there an existing API in Jenkins core or Stapler that exposes the resolved UI language for a page, rather than just the requested locale?

There is no such thing as a "resolved locale". Each individual resource is served in the requested language, or fallback (English). Basically all languages have incomplete translations, so parts of the page are English, others in the requested language.

[daniel-beck]

FTR this also doesn't address the requested enhancement as it doesn't add lang to

each passage or phrase in the content