"As much as it can". nftables and openvwitch both forward packets, and thus need to be written in C (or, perhaps, in the long term, Rust).
Really, you're playing on a semantic ambiguity in the word "router". A BGP implementation doesn't forward packets; it maintains a database of forwarding paths that the packet forwarding layer consults. In a large Cisco router, the SOC that runs BGP and maintains the RIB isn't the same electronic component that forwards packets.
