We used to present a variant of the main German stemmer, termed "german2" which was the same as the German stemmer except with the first step adjusted to improve handling of input text where the German letters ä, ö and ü, were written as ae, oe and ue respectively.
Snowball 3.0.0 added these adjustments to the main German stemmer, so there is no longer a "german2" variant - just use the "german" stemmer.