When the Khmer Rouge led by Pol Pot took over in 1975, the new regime "cleansed" the cities and killed everyone who were considered intellectuals or possessed skills, such las lawyers, doctors, teachers, scientists, public servants, etc. Even speaking a foreign language or wearing glasses got you killed.
Most embassies in Phnom Penh were shut down and the country remained largely isolated, but it still maintained diplomatic relations with a handful of countries such as China (its main backer), North Korea, Cuba or Egypt, among others, and was a member of the UN and the Non-Aligned Movement. The country also took part in international summits and hosted foreign delegations, whose visits were often reciprocated.
Designing and executing the foreign policy of a country requires a very particular set of skills, especially when it comes to language, international law and protocol, as well as years-long experience.
So, in the light of the foregoing, my question is: how did Democratic Kampuchea conduct its foreign policy in the absence of skilled people who could conduct it?
Thank you very much.
Unfortunately I do not have enough time to give this question a more detailed response so I hope the mods will forgive me.
So, with that being said...
Ieng Sary was a Senior member of the CPK 'Standing Committee', an intellectual, a Marxist-Leninist and by the time the Khmer Rouge took power an experienced diplomat after becoming the party's liaison officer with Hanoi in 1970. The generalisation of wholesale murder of 'intellectuals' is more readily applied to the wider Cambodian people following the liberation of Phnom Penh. It doesn't exclude the possibility of cadres within the party being trained as junior diplomats in a communist fashion leading up to the victory in 1975. That being said it was a tumultuous time in the early periods following evacuations of the cities etc but as you allude to in your question the CPK did end up setting up foreign offices for delegates from a bunch of sympathetic countries. Straight up communication with the party's upper echelons would be responsible for these early periods of contact and mostly with close neighbours; primarily with China, Thailand etc.
As for the specifics of how foreign policy was handled... well, the foreign diplomats in Phnom Penh were kept under heavy surveillance and not allowed very much freedom of movement. They were afforded a shop in their quarters of Phnom Penh that would provide them with goods. Foreign delegations that eventually visited the country once it had 'opened up' slightly were afforded similar restrictions.
The actual wheeling and dealing done by the CPK with other nations was handled by the more senior members of the CPK; Ieng Sary or Pot on most occasions with Khieu Samphan playing a larger role in the later years (Sihanouk until he was 'retired' also held some placeholder status in these kinds of events). The impression that there was no skilled people to conduct these kinds of deals doesn't quite map onto reality and Ieng Sary is the most obvious answer to your question in general.