fix(currency): stop showing dollars to Spanish users of the Android app

src/lib/currency.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { CURRENCIES, isValidExchangeRates, type Currency, type CurrencyInfo } from "./currency";
+import { CURRENCIES, currencyForLanguages, isValidExchangeRates, type Currency, type CurrencyInfo } from "./currency";
 
 describe("CURRENCIES", () => {
   it("is a non-empty array", () => {
@@ -94,3 +94,30 @@ describe("isValidExchangeRates", () => {
     expect(isValidExchangeRates("nope")).toBe(false);
   });
 });
+
+describe("currencyForLanguages", () => {
+  it("keeps EUR for a Spanish list that ends in en-US (Android WebView)", () => {
+    expect(currencyForLanguages(["es-ES", "en-US"])).toBe("EUR");
+    expect(currencyForLanguages(["es-ES", "es", "en-US", "en"])).toBe("EUR");
+  });
+
+  it("uses the first region's currency", () => {
+    expect(currencyForLanguages(["en-US"])).toBe("USD");
+    expect(currencyForLanguages(["en-GB", "en-US"])).toBe("GBP");
+    expect(currencyForLanguages(["de-CH", "de-DE"])).toBe("CHF");
+  });
+
+  it("skips languages without a region", () => {
+    expect(currencyForLanguages(["pl", "pl-PL"])).toBe("PLN");
+  });
+
+  it("reads the region after a script subtag", () => {
+    expect(currencyForLanguages(["zh-Hant-TW"])).toBe("TWD");
+  });
+
+  it("falls back to EUR", () => {
+    expect(currencyForLanguages([])).toBe("EUR");
+    expect(currencyForLanguages(["es"])).toBe("EUR");
+    expect(currencyForLanguages(["not a locale"])).toBe("EUR");
+  });
+});

src/lib/currency.tsx

@@ -91,16 +91,33 @@ const REGION_TO_CURRENCY: Record<string, Currency> = {
   AU: "AUD", NZ: "NZD",
 };
 
-function detectBrowserCurrency(): Currency {
-  if (typeof navigator === "undefined") return "EUR";
-  const langs = navigator.languages ?? [navigator.language];
+function regionOf(lang: string): string | undefined {
+  try {
+    return new Intl.Locale(lang).region?.toUpperCase();
+  } catch {
+    return undefined;
+  }
+}
+
+/**
+ * The first language that names a region decides: that region's currency, or
+ * EUR when the region is not in the map (Spain, France, ...). Later languages
+ * never override it. Android's WebView appends en-US to every list, so
+ * ["es-ES", "en-US"] must stay EUR.
+ */
+export function currencyForLanguages(langs: readonly string[]): Currency {
   for (const lang of langs) {
-    const region = lang.split("-")[1]?.toUpperCase();
-    if (region && region in REGION_TO_CURRENCY) return REGION_TO_CURRENCY[region];
+    const region = regionOf(lang);
+    if (region) return REGION_TO_CURRENCY[region] ?? "EUR";
   }
   return "EUR";
 }
 
+function detectBrowserCurrency(): Currency {
+  if (typeof navigator === "undefined") return "EUR";
+  return currencyForLanguages(navigator.languages ?? [navigator.language]);
+}
+
 // ---------------------------------------------------------------------------
 // Exchange rates (ECB publishes ~30 currencies; others won't convert)
 // ---------------------------------------------------------------------------