Hi !
I did not find anywhere a possibility to open web search or translations in a new tab instead of the current tab.
If it's not possible i think it would be a nice improvement , because when you want to translate a word in a page (right click) you always want to keep your current page active and not have to click on back to reload the page again.
It's already possible since long
Preferences -> Browsing -> Open [Selected Text] -> In a new tab
This will affect the "Open As URL", "Web Search" and "Translation" commands.