Add multilingual README support for zh-CN, ja, es

[SkyWuZJU]

What this PR does

Adds README support for Simplified Chinese, Japanese, and Spanish so readers of those languages
can follow your project just as easily.

This is additive and opt-in: no existing content or code is changed, and the
new files are safe to close or delete at any time.

Why we opened this PR

We're the team behind Tencent Hunyuan MT (HY MT2),
a translation model we recently open-sourced. We're using it to help open
source projects we appreciate reach readers in more languages.

We'd also love your feedback — both on the new READMEs in this PR and on the
model itself.

Scope of changes

New README files (with language-switcher links)

• README_zh-CN.md
• README_ja.md
• README_es.md

Existing READMEs updated with language-switcher links only

• README.md

That's the entire change: 4 files. Nothing else is touched.

Worried about the upkeep? A separate companion PR can help keep these
translations in sync automatically going forward — adopt it only if you
want to.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/vscode

Inputs:

• PR number: 2091
• Commit SHA: 6d2245fa02ebb61da1824ed8c01ff946f8a4cafd

Checks will be approved automatically on success.