Is it permissible for the government to borrow with interest from international bodies to finance essential projects, and is it permissible for a citizen to assist in that?

A loan that yields a stipulated benefit is usury (riba) and is impermissible by consensus, such as interest-bearing loans, which are forbidden by the Qur'an, the Sunnah, and scholarly consensus.

It is not permissible to borrow from banks or entities that deal in usury, nor to assist in such transactions, as this constitutes aiding in sin.