Run this AppleScript in an Automator service to open selected text in Google Translate.

translate.applescript

-- Modified from http://scriptingosx.com/2016/01/build-a-google-translate-service/
on run {input, parameters}
	set theText to input as string
	set oldDelims to AppleScript's text item delimiters
	set AppleScript's text item delimiters to {return & linefeed, return, linefeed, "
", character id 8233, character id 8232}
	--  list of line break chars from http://stackoverflow.com/questions/12546253/how-to-replace-n-in-applescript#12546965
	set splitText to text items of theText
	set AppleScript's text item delimiters to {space}
	set theText to splitText as text
	set AppleScript's text item delimiters to oldDelims
	
	tell application "Safari"
		activate
		set d to make new document with properties {URL:"https://translate.google.com/#auto/en/" & theText}
	end tell
end run

This seems to work if I select text in an application that is not Safari, but if I select text within Safari, it doesn't seem to work. I'm running Mac OS 11.1 Big Sur (Safari 14.0.2). Any thoughts about what to change to get this to work?

@applesean I'm running macOS 11.2 Beta 2 (20D5042d), so things may be a little different but: I used to be able to get this working by putting it in a new Automator Quick Action workflow but in order to get it to work in any app, I had to change the "Workflow receives current" option at the very top of the automator workflow to "text" instead of "Automatic". Since I changed it, I'v been able to auto-translate from any app including Safari. If that's not how yours is set to "text", would you give it a try?

If you tried it and it's still not working, can you tell me if it opens the Safari window to Google Translate and just shows nothing or if it doesn't open the Safari window at all?

So I was selecting text in the "Workflow receives current" menu. Actually, if you don't, then the Service doesn't show up in the Service menu. Strangely, that did not fix the problem – still a blank webpage when the workflow runs that opens on top of the Safari window that had the text.

I did hobble together an alternative script that is a bit short, and I'll paste here just for future reference, should someone need it. I still would love to know why this script doesn't load the Google Translate webpage in Safari, especially when the syntax checks out.

The simplified AppleScript for the QuickAction workflow in Automator that I've implemented as an alternative:
``

on run {input, parameters}
set phrase to input as string
set myBrowser to "Safari"
set toLang to "en"
tell application myBrowser
open location "https://translate.google.com/#auto/" & toLang & "/" & phrase
end tell
end run
``

No issue in macOS 11.3.1 with both the suggested apple scripts and the "Workflow receives current: text" option. Just make sure you create a Quick Action not a Workflow.