feat(metadata): add original-language preferences (#526)

* feat(metadata): add original-language preferences

* fix(settings): show metadata exceptions immediately

* docs(settings): add metadata language screenshot

* fix(settings): make language exceptions responsive

* fix(settings): standardize language display names

web/src/lib/languageNames.ts

@@ -0,0 +1,90 @@
+import "@formatjs/intl-displaynames/polyfill-force.js";
+import "@formatjs/intl-displaynames/locale-data/en.js";
+
+const englishLanguageNames = new Intl.DisplayNames(["en"], {
+  type: "language",
+  fallback: "none",
+});
+const englishScriptNames = new Intl.DisplayNames(["en"], {
+  type: "script",
+  fallback: "none",
+});
+const englishRegionNames = new Intl.DisplayNames(["en"], {
+  type: "region",
+  fallback: "none",
+});
+
+function displayName(names: Intl.DisplayNames, value: string): string | null {
+  try {
+    return names.of(value) ?? null;
+  } catch {
+    return null;
+  }
+}
+
+/** Canonical BCP 47 identity used only for comparison; wire values stay untouched. */
+export function canonicalLanguageTag(value: string): string | null {
+  const trimmed = value.trim();
+  if (!trimmed) return null;
+  try {
+    return new Intl.Locale(trimmed.replace(/_/g, "-")).toString();
+  } catch {
+    return null;
+  }
+}
+
+/** Stable identity that de-duplicates ISO aliases without collapsing script or region subtags. */
+export function languageIdentity(value: string): string {
+  return canonicalLanguageTag(value) ?? value.trim().toLowerCase();
+}
+
+/**
+ * Returns a deterministic English CLDR name for an ISO/BCP 47 value.
+ *
+ * Some CLDR releases omit a precomposed language-and-script label. In that
+ * case, compose the independently standardized language, script, and region
+ * names so tags such as `sr-Latn` remain distinct from `sr`.
+ */
+export function englishLanguageName(value: string): string | null {
+  const canonical = canonicalLanguageTag(value);
+  if (!canonical) return null;
+
+  let locale: Intl.Locale;
+  try {
+    locale = new Intl.Locale(canonical);
+  } catch {
+    return null;
+  }
+
+  const language = displayName(englishLanguageNames, locale.language);
+  if (!language) return null;
+
+  const exact = displayName(englishLanguageNames, canonical);
+  const qualifiers = [
+    locale.script ? displayName(englishScriptNames, locale.script) : null,
+    locale.region ? displayName(englishRegionNames, locale.region) : null,
+  ].filter((part): part is string => Boolean(part));
+
+  if (exact && (qualifiers.length === 0 || exact !== language)) return exact;
+  return qualifiers.length > 0 ? `${language} (${qualifiers.join(", ")})` : language;
+}
+
+/** User-facing name with an explicit fallback for unassigned or invalid tags. */
+export function getLanguageName(value: string): string {
+  const trimmed = value.trim();
+  if (!trimmed) return "Unknown";
+  return englishLanguageName(trimmed) ?? `Unknown language (${trimmed})`;
+}
+
+/** Empty-preserving variant for optional metadata and filter labels. */
+export function formatLanguage(value: string): string {
+  const trimmed = value.trim();
+  return trimmed ? getLanguageName(trimmed) : "";
+}
+
+/** Canonical ISO language subtag used for language matching and override keys. */
+export function normalizeLanguageCode(value: string | null | undefined): string {
+  const canonical = canonicalLanguageTag(value ?? "");
+  if (!canonical) return "";
+  return new Intl.Locale(canonical).language;
+}