BarTranslate

BarTranslate shows a trimmed-down Google Translate web view in a panel hanging from the menu bar. A configurable hotkey opens it with the cursor already in the source field, a second quick-translate hotkey translates whatever is on the clipboard, and the last-used language pair is remembered between launches.

Because translation happens on Google's web page, it needs an internet connection and depends on that service. The zipped app on GitHub releases is free; the developer also sells it on the Mac App Store, where it is notarized and updates automatically, as a way to support the project.