When you come to Germany, please purchase prepaid SIM as a contact method and put it on your smartphone.
There are many pre-paid SIMs now, but the same price range and traffic volume as Japanese cheap SIM.
However, carefully it depends on the type of carrier, LTE availability, fixed amount in EU area, etc.
In addition, I am using Ortel mobile. Monthly 9.99 €, 2GB, included in the EU flat-rate flat rate.
I think that the major Vofafone and O2 were similar conditions.
Furthermore, when you buy a prepaid SIM in Germany, you must confirm your identity.
Since you can verify your identity with Postident, let’s go to the post office with your resident card Meldebstätigung, passport. In that case, if Geburtsort is different from the passport listed passport, it is necessary to explain.