next-i18next: Memory leak caused by cloning the i18n instance

Describe the bug

The #1073 pr introduced a significant memory leak when using getServerSideProps.

Occurs in next-i18next version

v8.1.2

Steps to reproduce

1. using simple example from repo.
2. use getServerSideProps instead of getStaticProps in index.js
3. run yarn build on the example repo
4. start server in debug mode. In your package.json

"start": "NODE_OPTIONS='--inspect' next start -p ${PORT:=3000}"

1. In chrome open chrome://inspect url then click on inspect

2. Tab to the Memory tab.

3. Load localhost:3000. Take a snapshot (base).

4. Use loadtest or any other load testing library to hit localhost:3000 (2000 total requests). Send 1000 requests, stop the load, click collect garbage, take snapshot. Inspect the retained data.

5. Send another 1000 requests, stop the load test, click collect garbage, take a snapshot. Inspect the retained data. NOTE: it will take some time for the snapshot because of the tree created by the leak.

6. Observer the memory significantly increased and the GC never releases it.

7. Inspect the retained objects and you will see the i18n references.

Expected behaviour

After stopping the loadtest and manually triggering GC (or waiting for it to auto trigger) memory should go back to baseline (plus any additional memory used post first request). This is the behaviour in v8.1.1

Screenshots

As you can see the exact same test with the only difference being the version does not show a memory leak. In 8.1.1 we go from 11.3 -> 12.3 In 8.1.2 we go from 11.3 -> 66.3

8.1.2

8.1.1

OS (please complete the following information)

• Device: [MacBook Pro (16-inch, 2019)]
• Browser: [Chrome Version 89.0.4389.90]

Additional context

Add any other context about the problem here.