YouTube auto dubbed my videos into other languages, how do I switch it off?

Also worth knowing, viewers can already switch it back themselves: gear icon > Audio track > the original language. If you put a line in the description or pinned comment telling people that, it buys you time while you clean up the older videos.

It's on by default for some channels, that's why you never saw a switch. In YouTube Studio go to Settings, then Upload defaults, then the Advanced settings tab. There's an allow automatic dubbing checkbox, untick it and save. That covers future uploads. For the ones already out, open each video in Studio and check the Languages section. The auto-dubbed tracks are listed there and you can unpublish or delete them per video. Tedious if you have a big back catalogue, so start with the videos that get the most views from Brazil.