I found out yesterday that the German government provides a flowchart for developers wanting to calculate income tax
https://www.bmf-steuerrechner.de/interface/schnittstelle.jsp
However, this is the calculation for the monthly (pay as you go) wage tax and other deductions and not the yearly income tax.
There's a pdf version as well:
http://www.bundesfinanzministerium.de/Web/DE/Themen/Steuern/...
The problem is in computing the input values; it's quite hard to figure out without examples, except the most simple cases.