FefinDev/translator ? reverse-engineered prompt

Build me a simple Google Translate style web app with two versions, one in Flask and one in FastAPI, using the same frontend for both.

I want a clean, responsive page where I can type text, choose a source language and a target language, translate between many languages, swap the languages with one click, and auto detect the input language when needed. Make it look and feel close to Google Translate, with a nice modern UI and smooth behavior on both desktop and mobile.

Please set it up so each backend version works on its own, but shares the same HTML and CSS design. If you need to check current docs for translation libraries or best practices, go ahead and look them up online. Keep the app straightforward and easy to run locally.