Hi Venky Na,
I go through your post carefully and as per your description, based on to my test, I can get the same situation as you mentioned. Here are related screenshots for my test:
Mac: When I****Highlight cells Review>Translate>there is no Inset button
Windows: Same as Mac.
According to this Microsoft official article Translate text into a different language, in this part it notes that “In Excel, there is no Insert button, you'll have to copy/paste the text you highlighted in step1.”
By default, there’s no “Insert” button on Excel on Office 2019/365 for Mac in this case, you may refer above article in Mac version of Office.
We feel your pain due to this limitation and we’re really feel sorry for it. To address your concern about the situation, we will be very grateful if you could take a moment to send your feedback to our development team in the UserVoice forum. Your feedback and suggestions will help us continue to improve our product's features, thanks for your time and efforts.
I appreciate your patience and understanding.
Best Regards
Waqas Muhammad