Edit index.js or create a new file in the functions directory to add the translation function.
```javascript
const functions = require('firebase-functions');
const { Translate } = require('@google-cloud/translate').v2;

const translate = new Translate();

exports.translateText = functions.https.onCall(async (data, context) => {
const text = data.text;
const targetLanguage = data.language;

let [translation] = await translate.translate(text, targetLanguage);
return { translatedText: translation };
});
```

Use the callable function from your Firebase web app to translate text.
```javascript
async function translateText(text, targetLanguage) {
const translateText = firebase.functions().httpsCallable('translateText');
const result = await translateText({ text: text, language: targetLanguage });
console.log(result.data.translatedText);
}

document.getElementById('translateButton').addEventListener('click', () => {
const text = document.getElementById('textInput').value;
const language = document.getElementById('languageSelect').value;
translateText(text, language);
});
```