[Feature]: ADD language zh-cn

[Spc-jgs]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

add zh-cn

Proposed solution

add zh-cn

Why this matters

all chinese and use Chinese people

Smallest useful scope

add zh-cn

Alternatives considered

No response

Risks or tradeoffs

No response

Examples or references

No response

Contribution

• I would be open to helping implement this.

[juliusmarminge]

Closing as a duplicate of #6789, which already tracks Simplified Chinese localization.