Function detectLanguage

graph-ui/src/lib/i18n.ts:152–171

(acceptLanguage?: string | null, override?: string | null)

export type UiMessages = (typeof messages)[UiLanguage];

export function detectLanguage(acceptLanguage?: string | null, override?: string | null): UiLanguage {
  if (override === "zh" || override === "en") return override;
  if (!acceptLanguage) return "en";

  // Ranked by q, not by whether "zh" appears anywhere. A substring test served
  // Chinese for "en-US,en;q=0.9,zh;q=0.5", where English is clearly preferred,
  // and for "zh;q=0, en", where q=0 means Chinese is unacceptable.
  const best = acceptLanguage
    .split(",")
    .map((part) => {
      const [tag, ...params] = part.trim().split(";");
      const q = params.map((p) => /^\s*q\s*=\s*([\d.]+)\s*$/i.exec(p)).find(Boolean);
      return { tag: tag.trim().toLowerCase(), q: q ? Number(q[1]) : 1 };
    })
    .filter(({ tag, q }) => tag && Number.isFinite(q) && q > 0)
    .sort((a, b) => b.q - a.q)
    .find(({ tag }) => tag.split("-")[0] === "zh" || tag.split("-")[0] === "en");

  return best?.tag.startsWith("zh") ? "zh" : "en";
}

let cachedLanguage: UiLanguage = "en";
let languageLoaded = false;