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I thought I'd add my two-cents as I also recently had to fill in the tax questionnaire (I decided to start accepting donations).  I am individual from the UK, so obviously this does not address the German side of things, but might be useful to anyone going through these gruelling questions!

Same comment on the ITIN as pointed out by Wend1go.  TIN stands for Tax Identification Number and the questionnaire accepts your local country tax code.  In my case in the UK, this was my UTR (Unique Tax Reference).

Re-declaring income, I believe things work the sort of the same in the UK as Germany.  Assuming you are employed and game making is a hobby, there is a tax exemption for additional income made from side activities.  Best to check with an actual accountant, but anything under a thousand pound should be tax-free.  In my case I actually created a LLC with its own UTR.  I am self-employed and planning to have game development as a secondary income stream, so this made more sense for my situation (but was a bit more complex to setup!).

On a side note, I'd encourage anyone planning to actually selling game to set up a LLC, as it protects you as an individual.  Chances are very low, but some AAA company could decide to sue you for copyright infringement or whatnot (happened to notch in the early days of Mojang), so it's better they sue your company - not you!