When Safari detects content on a page in a language other than your default, it will offer a translation option. Click it, and the page will reload with the content translated.
- Mac: The translation button appears in the Smart Search field
- iPhone and iPad: The translation button appears in the menu on the left of the Smart Search field
The Mac’s menu allows you to mange preferred languages for translation. I can’t find this option in Safari on the iPhone and iPad, and it doesn’t seem to be in Settings either. According to Apple’s support doc, the available languages vary by country and region.