[fooofw]

My version of Firefox (146.0 on Debian) has exactly this. If I select a sentence and right-click, I get the menu item "Translate selection to <LANGUAGE>". In the resulting box, I can change the language pair - but the defaults that I have seen were also reasonable.

https://support.mozilla.org/en-US/kb/website-translation has the text: "A new Translate selection feature has been added starting in Firefox version 128, that enables you to highlight and translate selected text."

Edit: Sorry, I misread the comment to say that there was no such menu item. Edited to reflect this.