The App offers the Pro Version, but didn't Show the actual price

I don’t buy something, If I don’t know it and where the price isn’t transparent during the buying process. That also isn’t compliant to German law.

The price is fully transparent as all Google Play purchases …

Google does display the price to you before you accept, all purchases are made and handled by Google so I doubt Google would not be compliant to any law of any country they support purchases.