Actually its all over the place in whose guns they use.
Walther P99's are far more common and popular in Europe than they are in the states.
German Police do use HK's, Walther's and Sigs as well as Glocks.
From WIKI FWIW;
Walther- 41,000 P99DAO pistols purchased in 2005 for the North Rhine-Westphalia Police.[14][15]. The State police of Rhineland-Palatinate procurred approx. 10,000 units of the P99Q. The P99Q has also been ordered by the police forces of Hamburg (8,000 pistols), Bremen (2,000 pistols) and Schleswig-Holstein (8,000 pistols) with deliveries between 2009 and 2017.[16]
HK USP - Germany Bundeswehr[17] P8 and USP Tactical (P12)
German State Police forces[27] USP Compact (P10)
HK P30 - Germany: 13,500 pistols were purchased by the Federal Customs Administration.[2][5] 30,000 pistols were purchased by the Federal Police with an option for another 5,000.[4]
Sig 226 - Germany Spezialeinsatzkommandos (SEKs) of the police, and the Federal Criminal Police Office[11] P226
Glock - Germany GSG 9 of the German Federal Police[60]