When I right click in a text area, the menu is in english.
You are probably missing translation packages from your distribution.
I don't think there is a missing package, it's the only thing in english I can find. (I'm on Arch)
I'm using Kde Neon User Edition and I don't have this bug. It's probably a bug related to Arch packages.
Same on Tumbleweed. This is clearly a packaging issue.
Please report this problem to you distribution.
I'm on Arch too (updated today) and it's correctly translated.
Maybe a locale issue on your system ?
Damn, now I have no idea where this bug comes from... A config file which needs to be deleted somewhere?
This is a distro or configuration issue. Please ask in a forum or support channel for Arch.