Translation:Difficult languages are not greatly liked by the students.
Intuitively, "de" seems more natural to me, too. But then again that's probably due to my Romance language knowledge. And Esperanto is supposed to be neutral, isn't it. Did Zamenhof intend for "de" to be used as the passive agent preposition or is it merely a recent development?
Without trying to read the mind of Zamenhof, I'll tell you that "per" seems like a very odd choice to express the agent. By definition it expresses the instrument. If you have an example in mind with "per", I'd love to discuss it specifically. Otherwise, I think I've already given you the clearest answer I can think of -- use de.
That's weird, I was told by a long-time Esperantist that the passive voice agent is introduced through the preposition "per" and I simply swallowed it. However, I just checked the Vortaro and, guess what, it backs your claim 100 %. Sorry for the trouble and cheers for the help! :D
No trouble at all. That's why I'm out here answering questions.
For more help with Esperanto, check out my YouTube channel
"De" is used to introduce the agent, and "per" to introduce the instrument. The agent is one who would be the subject when you turn the sentence round: Students do not like difficult languages/Difficult languages are not liked by students. The instrument is used by the agent to perform the action of the verb "He was strangled with his tie" - so his tie is used to strangle him. This is introduced by "per" in Esperanto, "Li estis strangolita per sia kravato." You could also use "de", but in this case the tie is the agent and not the instrument: "He was strangled by his tie" - "Li estis strangolita de sia kravato". In English: "Li estis strangolita per sia kravato" = "He was strangled with his tie". "Li estis strangolita de sia kravato" = "He was strangled by his tie".