I don't like how translation is only unavailable when the browser "thinks" the whole site is in a particular language. What if there's a single sentence that's not? Or if it guesses the site's language incorrectly? No translation for you.

We need more control over the feature. Even just the ability to select text, right click, and have a "Translation" menu would be huge. Looks like there is such a feature, but it doesn't let you pick the language pairs, which is the most basic requirement of translation.

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.

before Firefox put it in the browser, the kinda finicky extension (which I still have installed) does in fact have this feature. highlight a work and you can translate specific passages.