(core) i18

Summary:
Adding initial work for localization support.

Summary in https://grist.quip.com/OtZKA6RHdQ6T/Internationalization-and-Localization

Test Plan: Not yet

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3633

app/common/gristUrls.ts

@@ -562,6 +562,12 @@ export interface GristLoadConfig {
 
   // If custom CSS should be included in the head of each page.
   enableCustomCss?: boolean;
+
+  // Supported languages for the UI. By default only english (en) is supported.
+  supportedLngs?: readonly string[];
+
+  // Loaded namespaces for translations.
+  namespaces?: readonly string[];
 }
 
 export const HideableUiElements = StringUnion("helpCenter", "billing", "templates", "multiSite", "multiAccounts");