"The butter is soft."
Translation:Die Butter ist weich.
"sanft" is a character trait. Somebody who is very careful and nice and speaks with a calm voice could be called "sanft"(gentle). That can be broadend to everything that gives you a similar feeling. Like a soft/gentle touch ("eine sanfte Berührung") or a gentle breeze ("sanfte Brise"). But butter can't be gentle on its own. It is soft in the sense of weak to the touch or "weich" in German.
I hope I could be of help.