those are completely different meanings; how would you use them to mean the same or have a similar meaning?

"Decisive" and "essential" have similar meanings, in that they both refer to something that is important. The difference is that "essential" describes something that cannot be done without, or an element or idea that fundamentally characterizes something else, in part or in full. "Decisive," on the other hand, describes an event which fundamentally shapes a later outcome. (Either that or it describes a person who makes swift, bold decisions, but I don't believe that's the meaning you're taking here.) Neither word is commonly used in an idiomatic sense that I can think of.

I have known Germans to use "decisive" in the sense of "essential" before, so there must be a good reason for it in terms of the German language, although I don't know what that reason is.