We've been collecting donations for about half a year now and it turns out that we receive about 50-60 Euros per month. As we're not charitable from the tax point of view, I also have to pay taxes on the donations like on any income. For that, I have to list all the donations including the donator, which is a bit of work. In the end, if you compare how much work that is and how much of the money is really left after taxes, I have to come to the conclusion that it just doesn't make sense to ask for donations. Thus, I'll remove the donations button from languagetool.org.
If you're interested in the donation history, see this Google spreadsheet.