Normal Switch controllers are sufficient, but we have no experience with tennis rackets.

The same thing happened to me ;-) The language in the game is German. The language depends on the console.

Yes, but there won't be all the features, like playing tennis by simulating a racket with the joycons (like on the wii) But I personally don't use this feature :)

Yes, the box might be different (not necessarily though) but the game is playable in both languages. 👍