feat: Language, the player's language as a BCP 47 tag

[LucasAntunesdeAlmeida]

What

g.Language() string: the player's language as a BCP 47 tag ("pt-BR", "zh-CN", "en-US", "de"), or "" when unknown. Games call it while building scenes to pick a translation.

Platform	Source
Windows	GetUserPreferredUILanguages(MUI_LANGUAGE_NAME) (the display language) via golang.org/x/sys/windows, no cgo; fallback GetUserDefaultLocaleName
macOS	first entry of defaults read -g AppleLanguages (Finder-launched apps get no LANG), then the environment
Linux / other Unix	first set of LC_ALL, LC_MESSAGES, LANG; pt_BR.UTF-8@euro becomes pt-BR, C/POSIX are ""; GNU LANGUAGE wins over a real locale, as for gettext
Browser	navigator.language

Every answer is normalized the same way (_ to -, language lower case, script title case, region upper case). The system is asked once per process.

Design choices

• A Game method, not a package function. "Agent or test run" is already one definition in the engine (g.agentOrTestRun(), which gates saves and audio), and it lives on the Game (Headless, Autopilot, DisallowAgents). New is the first line of every game, so the method is available before any scene exists.
• Agent and test runs get "". That covers Headless and Autopilot once called, COLLIDER_AGENT=mcp, and go test. The game falls back to its default language, and agents and CI read the same strings on every machine. mcp-window is a person's window, so it gets the real language, the same way it gets real audio and saves. Contract item 10.
• COLLIDER_LANG overrides it everywhere, agent runs included (COLLIDER_LANG=zh-CN go run ., or an MCP agent testing the Chinese UI). C forces "". It stays active after DisallowAgents: that lock stops outside control and file writes, while this only picks among the game's own strings, which a player can already do with LANG or the OS setting. It also lets testers check a translation on a release build.
• The name Language matches what games ask for ("which language?"). It returns a tag rather than a parsed struct so the API stays one string. The doc comment explains how to match it leniently (primary subtag, and region or script where it matters, for example zh-CN vs zh-TW).

Proof

• language_test.go: normalization table, the Unix env rules (priority, C/POSIX, LANGUAGE), AppleLanguages parsing, the agent/test/mcp/mcp-window/DisallowAgents matrix, the override, and the real platform call on the test machine (en-US here). language_windows_test.go covers the locale fallback.
• examples/dialog speaks English or Portuguese (COLLIDER_LANG=pt-BR go run .). Its typewriter now slices runes, not bytes, so accented letters never split.
• CI also vets and builds for Windows (language_windows.go), cross-compiled like the wasm step.
• README cheat-sheet row, contract item 10, AGENTS.md note, dialog README.