feat: Server side translation setup (#4657)

* Server side translation setup

* docs

server/utils/i18n.ts

@@ -0,0 +1,58 @@
+import path from "path";
+import i18n from "i18next";
+import backend from "i18next-fs-backend";
+import { languages } from "@shared/i18n";
+import { unicodeBCP47toCLDR, unicodeCLDRtoBCP47 } from "@shared/utils/date";
+import env from "@server/env";
+import { User } from "@server/models";
+
+/**
+ * Returns i18n options for the given user or the default server language if
+ * no user is provided.
+ *
+ * @param user The user to get options for
+ * @returns i18n options
+ */
+export function opts(user?: User | null) {
+  return {
+    lng: unicodeCLDRtoBCP47(user?.language ?? env.DEFAULT_LANGUAGE),
+  };
+}
+
+/**
+ * Initializes i18n library, loading all available translations from the
+ * filesystem.
+ *
+ * @returns i18n instance
+ */
+export function initI18n() {
+  const lng = unicodeCLDRtoBCP47(env.DEFAULT_LANGUAGE);
+  i18n.use(backend).init({
+    compatibilityJSON: "v3",
+    backend: {
+      loadPath: (language: string) => {
+        return path.resolve(
+          path.join(
+            __dirname,
+            "..",
+            "..",
+            "shared",
+            "i18n",
+            "locales",
+            unicodeBCP47toCLDR(language),
+            "translation.json"
+          )
+        );
+      },
+    },
+    preload: languages.map(unicodeCLDRtoBCP47),
+    interpolation: {
+      escapeValue: false,
+    },
+    lng,
+    fallbackLng: lng,
+    keySeparator: false,
+    returnNull: false,
+  });
+  return i18n;
+}