Translation:I am thinking about what it can do to oneself.
So, I take it that there is not a separate reflexive form of “én” in Danish, and that it is commonplace for “én” to take a reflexive role.
Duo’s English translation here feels badly constructed, because the reflexive pronoun “oneself” clearly does not refer back to the subject of its clause. If it did, it would have been “itself”. (But “itself” or “myself” is surely not the intended meaning by the indefinite pronoun “én” in the Danish.) It is puzzling what “oneself” could be referring to.
However, if the subject “it” referred to an action, then “oneself” could be understood as referring to the one performing the action. Now, it would be clearer if we made the action specific — say, “I am thinking about what smoking can do to oneself” (to borrow one of epac-mcl’s suggested scenarios). With contextual set up, “it”, or preferably “that”, can stand-in for “smoking”, and “oneself” can still be correctly understood — “Many of my friends are picking up smoking. I am thinking about what [doing] that can do to oneself.”
I wonder whether the Danish sentence also needs the help of a context to put “én" into a reflexive role. If so, then without the supporting context, could “én” just as well not be reflexive?
Talking about oneself in other languages can be confusing for English speakers because we have few instances of usage of these types of phrases in English. Example: It is nicer to say, "Talking about oneself is hard," than to say, "Talking about myself is hard." In many other languages you only say it the first way, but in English we usually use the second.