Freelance work for 2 weeks in Berlin

As a Spanish citizen there should be no legal problem to work in Germany. There are also tax agreements between all EU countries to avoid double taxation – although there might be a lot of paperwork involved. The real headache is the US. The IRS says all citizens, green card holders and resident aliens (anyone staying over 183 days in a year) are US persons for tax purposes. As a current resident of the US, they would theoretically require you to report such income to them as well.

Most Americans living abroad can avoid double taxation by taking the Foreign Earned Income Exclusion but you won't qualify for this for being abroad for just a couple of weeks. If you get taxed on this money in Europe you should be able to take a tax credit to offset any tax owed in the US. But for 2 weeks at a low pay job, the amount of tax on any earnings is probably minimal to nothing, depending on what other income you have. If you are earning income and having to file US tax anyway then it is no big deal to add this into the mix.

If these 2 weeks alone would mean you have to file then it adds a lot of trouble. I cannot suggest one break the law BUT in such a case one might weigh the trouble and cost against the strategy to simply not report it. It is not likely the US officials have the time or energy to find out that one worked for 2 weeks overseas. Generally one would say that this amount earned is under the filing requirements – except that when one is a freelancer they are thus self-employed and then accountable for American Social Security taxes from the very first dollar, or Euro, earned.

For just a two weeks engagement it is not worth the ffort to register as freelancer in Germany and go through all the legal loops (which will NOT be finished in that timeframe!).
But you are allowed to be engaged as a foreign freelancer, if you are properly registered as such in your home country. You also need to have (and state on each bill) your home country tax ID.

As Beepi mentions, seen purely practically, it is not worth registering. However, this entrepreneurial activity remains illegal. Sorry. Something else might be true for employees. Please consider your options.