macOS can already do this and almost nobody knows where the setting is. Here is the built in route, how to put a keyboard shortcut on it, and what to do in the apps where it quietly does nothing.
Highlight the text, control click it, choose Translate. That is it. macOS has shipped this since Monterey and it needs nothing installed.
It comes with a language picker, a Replace with Translation option that edits the text in place, and a speaker button. For a paragraph in Safari or a line in Mail it is genuinely all you need.
Open System Settings, go to Keyboard, then Keyboard Shortcuts, then Services. The translation entry is in that list, and you can give it any key combination you like. Reaching for the trackpad twice a paragraph is what makes people give up on the built in route, and this removes it.
If you want the result somewhere other than the little grey popup, the Shortcuts app can take the selection, URL encode it and open a translator with the text already in the field. Plenty of guides walk through that. It works, and it also means a browser window opens every time, which is the thing you were trying to avoid.
The Services menu is opt in. Native apps mostly support it, Electron apps mostly do not, so the same shortcut that works in Pages does nothing in a chat client and you are left wondering whether you pressed it wrong.
Then there is the bigger gap. Highlighting only works on text. A scanned PDF, a screenshot, a subtitle burned into a video, an installer whose buttons are all in a language you do not read: none of that is text, so there is nothing to select and no menu to open.
With Leafy running, highlighting text in any app pops up a small toolbar next to it, with Translate on it. Nothing to configure per app, nothing to remember.
For the text you cannot highlight, press ⌥A and drag a box over it. The screen freezes so nothing scrolls away, the words are read on your own Mac, and clicking Translate in the popup brings it back next to the original. That covers images, video, scanned pages, app interfaces and remote desktop sessions in one move. There is a longer page on that.
Translating a passage gives you the passage. Often the thing that stopped you was a single word, and a full translation smooths it over without ever telling you what that word was doing.
Tap the word instead. The sentence around it goes into the lookup, so you get the sense actually in play rather than the six the dictionary lists. Save it and the sentence is saved with it, into a list you can search, review and send to Anki later.
Selected text goes out to be translated, with nothing attached that says who you are. A box-select is recognised on your machine first, and the picture itself is never written to disk or sent anywhere.
Your word list stays local. No account, no sync server, no ads. The privacy page has the specifics.
Free while Leafy is in beta. One download, no account, macOS 14 and up.
Also worth a look: translating anything on screen, looking up a word on Mac, or a scanned PDF.