How do I restore my translate app, it got deleted by accident

Like any other app; go to the App Store, search for the app, then install it. If you cannot find it, search for the developer other apps (e.g. Pages), tap to see app description and scroll down to see ‘More by Apple’ and tap on ‘See All’ to get to Translate app.

Have you checked the App Library?

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.