Is this really an official english title for this movie?

Punctuation is funny and inconsistent in marketing for movies. Sometimes it is absent and other times not, usually because it's inconvenient or overlooked or because some rando just thinks it looks better without it. You can see it on the DVD from the UK here:

https://www.amazon.com/Singing-Ringing-Tree-Christel-Bodenstein/dp/B00004YS9R

But on others it is absent, like the Blu-ray from the UK.

But as the punctuation is in the German title, and the English title is the literal translation of the German, I don't see why it wouldn't be included.

I think you can add it as alternative title if both are in use

Sure, of course. But is the title without the comma an official English title?

You wrote that both titles are in use on DVD and Blu-ray so it's okay to add them