Selecting a piece of text, then right-clicking, navigating to the Kagi Translate extension and selecting any option does nothing on some sites. Checking the extension settings, I see that the statistics also says 0 words translated, so it really did nothing.
More specifically, I tried translating from Japanese to English on 2chan.net, and it doesn't work.
I then tried on Wikipedia on a Japanese article, works perfectly fine.
Tried using the default shortcut (Alt+T) instead, and this actually works fine.
Just installed the extension, so this is with default settings, on Vivaldi.
The Translate pop-up appears using the context-menu options.