Yeah weird, the description "in a way that is natural, often sudden, and not planned or forced" is exactly the meaning in German too. Maybe "spontaneous" has different semantics between US and UK English?
Words usually have multiple meanings, common ones and less common ones. "False friend" words usually swapped their common and rare semantics when entering a different language. In this case, the common meaning for "spontaneously" is "without direction" but not "on short notice". But it's the other way around in German. There actually is the German term "spontane Ordnung" ("order from chaos") using the English connotation in this case.
May I ask if you're a native German or English speaker?
There is an example in another thread here which has the following example:
We spontaneously decided to get married.
I would translate that into:
Wir haben uns spontan (dazu) entschieden zu heiraten.
What is the difference in these two? Same words, same meaning to me as a native German speaker.
As I see it, spontaneously can have a subtle other meaning than spontan, but can also be the same.
But we can resolve this confusion around "spontaneously" by going to its actual origin: The English word "spontaneous" is a loan word and originates from the Latin "spontaneus" which itself derives from the Latin noun "spons" (feminine) primarily meaning "free will". It also has a secondary connotation meaning "impulse". [1]
So the common English usage is closer to how the Romans used the word. That does make sense: The Romans conquered the British isles but failed to win against the Barbarians that lived east of the river Rhine. The Germans only much later incorporated Latin vocabulary and did so mostly only in elite circles. That's possibly why "spontan" means "acting out of impulse".
FWIW I usually use "spontan" like "ungeplant" (unplanned). Maybe there's even differences for native speakers in different parts of Germany (wouldn't surprise me, especially between East and West).
This is what I was confused about as well. In the example the meetup itself could be held as non-spontaneous (telepaths spare the price of the phone call), but deciding on the place will still be done on a whim, off the cuff, impromptu, not before, impulsively. Spontaneously? :)
(On an unrelated note, let me share my favorite Anglicism that I've heard in a movie: "gekidnapped")
