Add i18n / locale support for SpecKit

[Nicered]

Hi everyone,

While experimenting with SpecKit, I noticed that the English-only flow can be a barrier for developers who are not fluent in English. In Spec-Driven Development, specs and plans are meant to be the "single source of truth," but when teams translate them informally, meaning drift and inconsistent terms often occur.

I tried this in PR #657, which added YAML-based locale files and a --lang option. That PR was closed with the suggestion to discuss the approach first — so here’s a starting point.

Why it might help

• Reduce misunderstandings by letting specs/plans/tasks be written in native language
• Make adoption and onboarding easier for non-English teams
• Keep translations aligned with English source via CI checks
• Encourage more community contributions (docs, guides, translations)

Possible scope

• Locale files: locales/en.yaml (source of truth) + locales/ko.yaml etc.
• CLI option --lang or config to select language
• Fallback to English if keys are missing
• Start small: only /specify, /plan, /tasks prompts