Replace hard-coded localization file with a dedicated i18n library

[hongqitai]

Problem

The localization strings in the current project are centralized in crates/tui/src/localization.rs. This file has exceeded 5,000 lines and is entirely hard-coded, which may lead to low maintainability, affect compilation speed, and create difficulties in using translation toolchains.

Proposed solution

Consider introducing a mature Rust i18n library, such as fluent-rs or rust-i18n, and moving the localization translations to dedicated localization files.

Impact

• Increased dependencies: Adding 2–3 new crates, but they are mature and well-maintained. This will slightly increase build dependencies, yet the benefits far outweigh the costs.
• Performance overhead: Most i18n libraries have been thoroughly optimized, so the performance difference compared to hard‑coded strings should be negligible.
• Migration effort: The workload is relatively large and may introduce logic changes, so a detailed refactoring plan needs to be discussed.

[cy2311]

Started implementing this with Step 1: extracted Chinese Simplified translations into a dedicated JSON file.

PR: #3549

• locales/zh-Hans.json (new) — 408 Chinese translations
• localization.rs reduced by 541 lines

Next steps:

• Extract remaining languages
• Evaluate fluent-rs or rust-i18n
• Add CI checks for coverage

Implemented by @cy2311

[Hmbown]

thank you all so much!! this is so helpful.