Add Vietnamese (vi) to voice dictation supported languages

Description

Voice dictation currently supports 20 languages (cs, da, nl, en, fr, de, el, hi, id, it, ja, ko, no, pl, pt, ru, es, sv, tr, uk) per the Voice dictation docs, but Vietnamese (vi) is not included.

Setting /config language=vi successfully changes Claude's response language ("Set Language to Vietnamese"), but voice dictation silently falls back to English since vi isn't in the supported dictation list — this is confusing because the same language setting controls both response language and dictation language, yet only one of the two actually respects the Vietnamese value.

Environment

• Claude Code CLI: 2.1.139
• Claude Code VS Code extension: 2.1.221 (darwin-arm64)
• OS: macOS (Darwin 24.1.0)

Steps to reproduce

1. Run /config language=vi — confirms "Set Language to Vietnamese"
2. Use voice dictation (mic button in VS Code extension, or /voice) and speak Vietnamese
3. Dictation output is in English instead of Vietnamese, with no visible warning that vi isn't supported for dictation

Expected behavior

Either:

• Add Vietnamese to the voice dictation supported language list, or
• If not feasible short-term, surface a clear warning when language is set to a value not supported by dictation, so users understand why dictation isn't matching their configured language (related to #69936, which proposes separating STT language from response language settings)

Why this matters

Vietnamese speech recognition is a well-established capability across many other AI/voice products, so this is understood to be a product scope gap rather than a technical limitation. Given the size of the Vietnamese-speaking developer community, adding vi support would help close this gap.