In principle the Translate button is supposed to be used only to create a NEW translation. But lots of people (including myself) tend to click on it for many other things like:
When looking at French that is undergoing translation from English, they click on Translate button to continue that translation.
When looking at the English page that needs an updated translated to French, they click on Translate button to initiate or continue that translation.
This tells me that the Translate button should behave more like a menu. When you click on it, you get a list of choices: