This is the entry in Schaefer's Wirtschaftsenglisch:
Steuerausländer m
(StR) nonresident individual (ie, person subject to limited tax liability)
Of course, it explains rather than translates. I would use tax-exempt foreigner.

I would choose fiscal alien,
as a Steuerausländer one si not necessarily exempt from taxes. For instance, if you are Steuerausländer under German law, and have taxable assets in Germany, you are still liable for taxes, but often at a different rate. Alternatively, you should/could (have to) pay taxes for this in the country you reside in, for foreign assets, if the two countries have a mutual tax treaty.