The UEFI Forum’s Tianocore implementation is written in C, with a bit of assembly language. The U-Boot EFI implementation is in C. Most operating systems are written in C (although I hear that the current Windows kernel has been (or is being) rewritten from C to C++). There are a few C++/UEFI projects on Github, including a few C++-friendly versions of the UEFI Forum’s C-centric core header files. And I just noticed there was a talk at CppCon18 on this topic, video and slides below.
PS: To someone with a bit of spare time, who has an interest in UEFI and C++: please port Nmap to UEFI. Thanks!