Example of caching Google Translate translation results for a multilingual site on NextJS

I am sharing my experience organizing the translation of content into different languages. I work on a multilingual website using the Next.js i18n + MongoDB (Mongoose) stack. The site contains a significant amount of text that is occasionally updated.

Google Translate is used to translate the texts in the project. Initially, a simple function was created to translate the view.

// @/lib/translate.ts

import "server-only";
import { z } from "zod";

const { Translate } = require("@google-cloud/translate").v2;

const ApiKey = z.string().parse(process.env.GOOGLE_TRANSLATION_API_KEY);
const googleTranslate = new Translate({ key: ApiKey });

export async function translateFromEn(text: string, locale: string): Promise<string | null> {
  try {
    if (locale === 'en') return text.trim(); 
    const translations = await googleTranslate.translate(text, locale);
    const result = translations[0];
    return result;
  } catch (error) {
    return null;
  }
}

Then it was used everywhere when preparing pages. To avoid translating the same texts repeatedly, I used the "unstable_cache" function (despite its name, I've never had any problems with it).

// .../page.tsx

type Params = { locale: string };

export default async function Page({ params }: { params: Params }) {
  const {  locale } = params;

  ...

  const translatedTitle = await unstable_cache(
      async () => await translateFromRu(post.title, locale),
      [`post:title:${post.slug}:${locale}`],
      { revalidate: false, tags: [`post:${post.slug}`] },
  )();

  const translatedShortDescription = await unstable_cache(
      async () => await translateFromRu(post.metaCustom.shortdescription, locale),
      [`post:shortDescription:${post.slug}:${locale}`],
      { revalidate: false, tags: [`post:${post.slug}`] },
   )();


    return {
      ...
    }

  }

Accordingly, whenever the blog post was updated, the revalidateTag() method was called and the translations were updated.

While this was a working solution, the cache was reset every time the site was updated, which was a flaw. Consequently, this risked a large bill from Google.

Therefore, I added a feature that stores the translation results in the Mongo database. The idea is as follows:

1. Create a unique ID for each translation entity.
2. Before accessing the Google API, we check the database to see if there is a record with this ID and if it matches the text to be translated.
3. If so, we return the record from the database.
4. If not, we request a translation via the API, return the result, and save it in the database.

I use hashing for simple text matching. With the new logic, the translateFromEn function looks like this:

import "server-only";

import crypto from "crypto";
import { z } from "zod";
import { TranslateResult, zTranslateResult } from "../mongoModels";
const { Translate } = require("@google-cloud/translate").v2;

const ApiKey = z.string().parse(process.env.GOOGLE_TRANSLATION_API_KEY);
const googleTranslate = new Translate({ key: ApiKey });

// -------------------------------------------------------------------

type GetTranslationKeyProps = {
  locale: string; // en, ko, ch...
  entity: string; // user, post, comment
  entityType: string; // shortDescription, metaTitle
  entityId: string; // slug, _id ...
};
function getTranslationKey(props: GetTranslationKeyProps) {
  const { locale, entity, entityType, entityId } = props;
  return `${locale}-${entity}-${entityType}-${entityId}`;
}

// --------------------------------------------------------------------
// 1. If locale === 'en' - just return text
// 2. Generate Key and Check if translation exists in DB
// 3. Generate hash from text
// 4. If old Db Translation exist and hash is the same - return translation
// 5. If hash is different or hash not the save - create new translation
// 6. Return translation
// --------------------------------------------------------------------

export async function translateFromEn(
  text: string,
  idData: GetTranslationKeyProps,
): Promise<string | null> {
  try {
    // 1. If locale === 'en' - just return text
    if (idData.locale === "en" || text.trim().length === 0) {
      return text.trim();
    }

    // 2. Generate Key and Check if translation exists in DB
    const key = getTranslationKey(idData);
    const oldTranslateDoc = await TranslateResult.findOne({
      key,
    }).lean();

    // 3. Generate hash from text
    const textHash = crypto.createHash("SHA-256").update(text).digest("hex");

    // 4. If old Db Translation exist and hash is the same - return translation
    if (oldTranslateDoc && oldTranslateDoc.hash === textHash) {
      return oldTranslateDoc.translation.trim();
    }

    // 5. If hash is different or hash not the save - create new translation
    const translations = await googleTranslate.translate(text.trim(), idData.locale);
    const translateResult = translations[0];

    const newTranslationResult = zTranslateResult.parse({
      key,
      hash: textHash,
      translation: translateResult,
    });

    await TranslateResult.findOneAndUpdate({ key }, newTranslationResult, {
      upsert: true,
    });

    // 6. Return translation
    return translateResult;

    // ---
  } catch (error) {
    return null;
  }
}

This allowed us to significantly reduce the number of calls to the Google Translate API and stay within budget.