Constraint: Language (BCP 47)

[cursor]

Goal

Add a Language constraint — validate a string as a correct BCP 47 language tag / application-allowed subset (like Assert\Language in Symfony).

Reference

• Symfony: Symfony\Component\Validator\Constraints\Language.

Constraints

• No new dependencies in go.mod — golang.org/x/text/language is allowed (already a project dependency): use language.Parse and optionally compare against an allowlist.

Scope

• Error messages, tests: valid tags, garbage input, optional list of allowed languages (if present in Symfony).

Complexity

2 / 5.

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Migrated from Linear: IGO-40
Priority: High
Estimate: 2 points