If it's a purely learning project for myself I would usually pick a new language which I would like to learn. If it's not, you owe it to your client to have an

If it's a purely learning project for myself I would usually pick a new language which I would like to learn. If it's not, you owe it to your client to have an open discussion about which backend (and frontend) language to select. If you are working with a contract, remember that you are only selling your services as a software engineer and that the client will eventually own the code (unless there is some other agreement).