[lasfter]

Without any prior forethought. Immediately, on a whim.

E.g., "We were in Vegas and saw an Elvis chapel, so we spontaneously decided to get married!"

They were in Vegas without any plans of marriage, saw a venue, and decided immediately to marry.

[ThePhysicist]

That seems like exactly the meaning that "spontan" has in Germany. It's like you plan to be in Vegas but there you spontaneously decide to get married. In German, you would say "Wir sind nach Vegas in den Urlaub geflogen und haben dort spontan geheiratet", so pretty much identical.

[kergonath]

Something spontaneous is something that happens on its own. If you can say that you will do something, then it is not spontaneous. You cannot decide to do something spontaneous next week because then you’ve planned it. Improvised may be better suited in that case, from what I understand. It’s similar in French: “il y aura un rassemblement spontané” (there will be a spontaneous gathering) is nonsense. Even if it is understandable, it would label you as a foreigner or uneducated.

Similar words generally have different scopes in different languages, or have more meanings in a language than in another. So you have to go further than the first meaning in a dictionary to make sure that a word is right in a given context.

For the sake of the argument, if we assume that a word with one meaning in English has the same meaning in German plus another one, then using it with the second meaning in English is wrong. Native speakers are very sensitive to words being slightly wrong or use in slightly wrong places. It happens all the time with foreign languages, and I am sure a native English speaker could point out one example in this very post.

[tarkin2]

Yeah. The article is confused.

In its example deciding spontaneously is fine since you're not deciding what to decide but you're deciding that you will decide and then later you'll suddenly decide what to decide. The decision to decide was planned but the substance of the decision is yet to be decided.

[zamnos]

That's different than vanderZwan's quoted description of what spontan means in the GP comment.