Looking in the App Store, it seems the iOS version of Machinarium was updated on October 6, 2016, with the ability to switch between several different languages. You can switch languages from the black title screen with the white Machinarium letters—the one that displays when the game is first launched. (So if you have the game running, you'll need to quit it and relaunch it in order to see that screen.) Here's a screenshot:
Notice there are arrows at the bottom of the screen to switch languages...keep tapping and you'll get back to English.
Edit: Just an update to say that Machinarium on iOS currently has the ability to switch the language from the menu: