I couldn't give you a grammatical rule, leeneia, but I'd stick with "Wenn der Frühling kommt..." All the German recordings have "wenn" (if). I wouldn't correct them. "Als" would work, as in the Dutch, but I'll leave the "wenn."

Here's leeneia's MIDI. I sped it up a bit and removed the sharp in measure 33.

Here's another German recording, by Mieke Telkamp:

https://www.youtube.com/watch?v=X22vAmpZSdY

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Click to play (joeweb)