Instructions are there on the backdrop. Click the green flag first. You need to wait a few seconds, a minute at most, between entering a sentence and getting asked to enter another sentence. The longer your sentence is, and the more languages you choose to cycle through, the longer you have to wait.
After seeing that you can translate stuff with Scratch 3.0, this idea immediately came up. You can use it to translate songs, terms and conditions, recipes... anything! The results get incredibly mangled though. But that's the fun! For some reason, the first two lines get deleted. Any better Scratchers care to remix and fix the problem? Or I could figure it out in my own time, but that'll take a while...