Purchase US GSM phone for use in Germany?

[inquisitor]

The most common UMTS frequency band in Europe is called UMTS2100 or UMTS frequency band I and uses 1920-1980 MHz for the uplink and 2110-2170 MHz for the downlink. Some European operators additionally use UMTS900, however none of the German ones.

AWS which is also known as UMTS1700, UMTS1700/2100 and UMTS frequency band IV uses a way lower frequency for the uplink (1710-1785 MHz) but shares a subset of the UMTS2100 downlink frequency range for its downlink.

For an overview of UMTS frequency bands see UMTS frequency bands - Wikipedia, the free encyclopedia

The overlapping downlink frequency ranges of UMTS2100 and AWS facilitate the integration of UMTS2100 in AWS-handsets. So most AWS handsets also support UMTS2100, but not the other way around.

According to Wikipedia and several other sources the T-Mobile USA version of the Galaxy S2 (also know as SGH-T989) supports UMTS2100:

The cellular radio of the T-Mobile supports UMTS bands I (2100 MHz), II (1900 MHz), IV (1700 MHz) and V (850 MHz).

So the Samsung SGH-T989 will work in Europe provided that it is unlocked.