"She eats desserts on plates."
Translation:Ea mănâncă deserturi pe farfurii.
pe = on
în = in
For having something on your plate, it can be argued that it is both in, in some sense, as well as on, so Romanians use both.
Well, for example, if it is soup we say: supa e în farfurie, instead of pe farfurie. But for most other things, indeed it is more common to say pe farfurie.