(core) Simple localization support and currency selector.

Summary:
- Grist document has a associated "locale" setting that affects how currency is formatted.
- Currency selector for number format.

Test Plan: not done

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2977

app/server/lib/ServerLocale.ts

@@ -0,0 +1,16 @@
+import {parse as languageParser} from "accept-language-parser";
+import {Request} from 'express';
+import {locales} from 'app/common/Locales';
+
+/**
+ * Returns the locale from a request, falling back to `defaultLocale`
+ * if unable to determine the locale.
+ */
+export function localeFromRequest(req: Request, defaultLocale: string = 'en-US') {
+  const language = languageParser(req.headers["accept-language"] as string)[0];
+  if (!language) { return defaultLocale; }
+
+  const locale = `${language.code}-${language.region}`;
+  const supports = locales.some(l => l.code === locale);
+  return supports ? locale : defaultLocale;
+}